Do what bigtav said, before working with weights, I used to do pushups and situps everyday. This was when i was about 10-11 though, but they can still benefit someone just starting out.

When you're ready, figure out a way to cut costs in order to afford a gym membership. Most people can find a way to save 1$ per day which is really all a gym membership costs. whether its finding cheaper foods, not going out, drinking etc...just think, 1 day out for dinner in a month is more than a monthly gym membership.

Also, if money is really tight, the YMCAs will offer a reduced rate, as long as you aren't taking advantage. Once your committed, money will never be more important than health.