my favorite type of music is heavy metal 
Metal takes influence from many genres such as blues rock. It's also known for it's overall loudness and amplified distortion.
do you know when and where heavy metal actually started?
Yes, actually. It developed in the late 1960s and early 1970s, largely in the United Kingdom, believe it or not.
to me heavy metal started with Black Sabbath but I guess there were bands before them
You're right that there were other bands, but Black Sabbath was influential as well. They formed in Birmingham in 1968
other than Black Sabbath, I cannot think of any other band during that time period
In addition to Black Sabbath, two other famous acts, Led Zeppelin and Deep Purple were founded.