Your budget is limiting you, for sure. Don't forget that you will need to also buy some protective equipment for yourself (helmet, sturdy boots, gloves and pants at a minimum). Or maybe you already have some stuff that will work for a while. It is not impossible to find a decent bike for $800.00. About double that would be better, unfortunately. You will just have to look harder, that's all. Try to find someone who is very mechanically-inclined to come with you (and also to curb your enthusiasm a little so you don't bolt and buy a P.O.S.). You should be looking at 250 enduro 2 strokes like Kawasaki KDX200/220's, or older Yamaha WR250's (probably early to mid 90's stuff). There are also some 4 stroke choices that would be good. Such as Honda XR400's. Personally, I think you should look for a KDX200. There are lots available, they are a good, reliable trail bike. And they won't scare you too badly with power. Find one of those in good shape and it would be a really good start for you. Take your time, look around. Buy something that looks like it is well looked after, not something filthy and missing parts.
2c