I was in the same spot your'e in now -- hadn't been on a bike in a couple of decades, at least! Had NEVER been on a bike with handbrakes or gears, and was really worried about that.

The bike shops I visited let you ride on the parking lot. That was really good for me. The guy who helped me the most had me go round and round and do some figure 8s, then go the other direction -- he had me go over speed bumps in the parking lot, then "take that a little faster this time," etc. He was watching closely the entire time and calling me over to make adjustments, but for me, it built my confidence. Even having him direct me to ride over a particular bump or crack in the lot gave me confidence, because it told me, "Oh, I guess it's okay to do that -- it won't make me flip or anything...!"