Just ask among your friends, you will find a lot of them are interested in riding bike but don't know where to start. All you need to do is ask around.
By taking it to a bike shop for a tune-up, they will check out the brakes, shifters and make sure everything functions properly.
One of the bro in the forum ara03kw seems to know where you can put up your used bike for sale. I hope he can answer your question.