Playing the piano is impressive. A suggestion for improving dexterity is practicing scales at increasing BPM's. It helps to build speed and dexterity.
What are BPM's? I find scales boring, is there a more exciting way to improve finger dexterity?
BPM is beats per minute, or tempo. Youtube has a good video on playing songs to help with dexterity , https://www.youtube.com/watch?v=cC5rmzqoSII  the title is Velocity Exercises For Piano by liberty park music
I see. The problem is that I do not have access to You Tube. I remember there was a book by Czerny form the School of Velocity series called The Art of Finger Dexterity. Do you think that might be useful?
Yes that is a great book. Czerny was a great musician from the 1800's and there are some great tips in there on hand placement and specific exercises for beginners to help strengthen bot the left and right hands. While scales are not exciting that is also something that is in the book.  
Cool, I will get the book. I don't think there are pure scales, but they are disguised within the different etudes. My goal is to be able to play the third movement of the Moonlight Sonata... but it's so difficult. Is there any food I can eat that will be good for my finger dexterity?
Nutriotion is vital for the body to function properly. Eating a balanced mix of fruits vegtables and protein will give your body the vitamins a nutriants to allow your body to perform at it's peak. More specifically Fish which is high in Omega fatty acids would be a very beneficial food to help your body play the piano at it's peak.
Ah good, I will eat more fish. What about acupuncture, can that help with improving finger dexterity so that I can achieve my goal of playing the third movement of the Moonlight Sonata by Beethoven?
Acupuncture has benefits in pain relief and can help re channel the flow of energy in your body. While there is no direct benefits linked to dexterity, it could possibly help with your overall goal of playing the third movement of the Moonlight Sonata.
Ok good, I will book a session. What about the pianoforte, which is the best type of pianoforte for playing fast pieces such as the third movement of Beethoven's Moonlight Sonata?
There are many different piano builders out there. Reviews are mixed on what is the best one for fast pieces. Overall the best type of piano is the one you feel the most comfortable on. Piano purchases are normally once in a lifetime so it is best you get one the suites you. A couple brands in the top 10 are Bechstein and Steinway. 
Ok I will look at them. Do you think it is better to play by reading the sheet music or memorizing the piece, and in particular the third movement of the Moonlight Sonata?
Memorizing a piece is hard work and by memorizing your build muscle memory. The muslce memory is crucial for playing a fast piece like the Moonlight Sonata. Your fingers need to know where they are going on the piano keys before your eyes are reading the next notes.
So that would mean that I need to work on my memory too? Are there any memory exercises that I should be doing? Also I heard that there are some pills you can take to improve your memory, is that true?
Omega-3 fatty acids are also good for memory, then B vitamins are also known to help memory and performance.  Memorizing a piece comes with practice and playing through the piece numerous times. As they say practice makes perfect.
Would I be able to play faster if I did not look at the keys? Maybe by trying to play blindfolded?
Mozart was known to play blindfolded and he is one of the best piano players in history. Playing blindfolded is a good practicing technique and can improve the speed at which you play.
Oh good, I will get a blindfold. Do you think a digital piano is as good as an acoustic piano for playing fast pieces such as the third movement of the Moonlight sonata?
It is difficult finding the right piano. While digital piano's are easier to maintain and are cheaper, there is no replacement for the sound of an acoustic piano. The Yamaha Avantgrand is a digital piano professionals use and might be worth trying out to see if you like it.
Ok then I will stick to acoustic pianos as I want the third movement of the Moonlight sonata to sound as good as can be. Is it a good idea to try and play alongside a recording or would that be confusing?
Practicing playing the piano has many different techniques. Utilizing a recording is a way to practice but as you said it can be confusing. However, listening to the piece while playing it may help you recognize where you are struggling to keep up with the pace of the music or hit a wrong note. It definitely is another tool to use to practice your mastery of the Moonlight Sonata.
Good, I will try practicing along with a recording of the Moonlight Sonata. I also wonder if it helps to be in the right mood before practicing, such as doing some kind of relaxation exercise. Would aromatherapy be useful to de-stress before sitting at the piano?
Essential oils like lavendar have been used for many years in attempt to improve a persons mood. Having a clear mind is a good thing when playing piano. It is also good not to be overstressed when playing. It is best to approach the piano as something that is fun and not a job. It will let you relax and perform the Moonlight Sonata to its fullist extent.
Ok I will stock up on essential oils. Also should my piano face into the light or away from the light? Sometimes I like to face the window while playing but it can also be distracting, specially if I am trying to focus on achieving my ambition of mastering the third movement of the Moonlight Sonata.
The locating for setting up your piano may affect your play. Distractions are not good and could cause you to play the wrong notes. If your goal is to be a concert piano player, it might be a good idea to get used to playing with distractions and with the light. Professional musicians play in high lights and have many distractions. If you are playing for recreation and just for yourself, it is advisable to set the Piano up where you are most comfortable including adequate lighting to be able to read the music and less distractions so you can focus on the task at hand. 
Okay I will take your advice. What about the ambient temperature, should I put on the air conditioning to have a cooler room or the stove to warm up the room, which is better for playing a fast piece such as the third movement of the Moonlight Sonata?
It is best to play in a room where you are not near a heat or cold air source. Constant changes of temperature can reduce your abilities as your body adjusts. About 70 degrees F is the reccomended comfortable temperature.
Ok I will set my thermostat to 70 F. I also heard it is not advisable to play on a full stomach because your blood flow is diverted to your stomach and you feel sleepy. But I don't think it would be good to play when you're hungry is it? I get irritated easily if I'm hungry and I might get irritated if I'm not hitting the right notes. What I should I do?
Eating while overly full is not the best idea as your body does spend energy digesting and does divert blood flow to the stomache to aid in the process. However, not having enough food in your body to perform properly is also a problem. If you are hungry before playing the piano a light snack like yoghurt or granola would be reccomended to help you play without being dampered by being hungry.
Okay, I will stock up on yoghurt and granola.Also I spend a lot of time using a computer mouse, do you think this will have a detrimental effect on my developing the degree of finger dexterity I need in order to play the third movement of the Moonlight Sonata, which is after all my goal ? If I should stop using a mouse, is there any other alternative for doing my computer work?
A ergonomic mouse would help keep your hand relaxed while using the computer. A vertical style mouse can be purchased on Amazon, Logitech and Anker are two brands that have vertical mice. Evoluent is a brand that is more expensive and makes both left and right handed vertical mice that keep your wrist and hands in optimal positions to reduce strain and fatigue.
Oh that's good, I will order one immediately! Also you said before that practice makes perfect... but what can I do to motivate me when I'm feeling too lazy to practice? Do you know any good tricks for motivation?
There are a lot of different reasons on why we are motivated to do things. A good way to stay motivated is to have a plan, if you are having trouble staying motivated to learn Moonlight Sonata, break it down into smaller pieces. Maybe practice only 1 page a day. If you still get burnt out practicing or do not feel motivated to practice, maybe play a song you already know and enjoy to get the momentum of playing going and hopefully that carries over into practicing Moonlight Sonata. 
Ah that is a good idea, I will set myself little goals instead of trying to learn the whole piece at once.  I can't wait to try out all your tips! Thank you!
You're welcome. Hopefully they help you. You asked a lot of good questions today and that shows you are dedicated at achieving your goal of playing Moonlight Sonata. Keep up the good work!