I would ask your future class instructor for his/her advice. They can look at your physical appearance and perhaps recommend something to start with. Frame size and tire/wheel size etc. If your not comfortable you will not want to ride.
Maybe that Walmart bike isn't such a bad idea for some one just starting out and learning.
Nothing wrong with a single speed for starters.
If you decide you like bike riding, you can move on from there. If you decide it's not for you, not much lost.
You can give it to Good Will.