If the bike is really worth ~$100 then it's probably going to need several hundred dollars of work before hitting the trails. New BB, new cassette, new chainrings, new chain, new cables, new brake pads, rebuild headset, perhaps new wheels/hubs...a lot depends on the specific condition of the bike. Riding a crappy/poorly maintained bike is not an enjoyable experience.
On the other hand, if the bike is in pretty good condition with decent components then it's probably worth a lot more than $100 and a good chance it's stolen.
My recommendation: Start off by renting bikes at your local bike shop. This will let you try out different bikes...different brands, sizes, and features. If you decide you like the sport then you should be able to get into a quality new or new-ish hardtail mountain bike for a little less than $1000.