A couple of quick tips before you enter your first contest: first, while you are starting out, don't enter any contests that require any kind of entry fees. There are prestigious photography contests out there with very significant prizes that do require entry fees, but when you are just starting, it's better if you stay away from the expensive ones. There are more than enough free-to-enter online contests out there for you to enter, and some of them actually do offer a small cash prize for winning. Above all else, when you are just starting, it's more important for you to gain some exposure, tips and photo ideas from other photographers.
The most important rule to follow when entering online contests is pretty simple: have fun! The great thing about contests is that if you don't win one, you can enter your photograph in as many more as you want. With some hard work and tenacity, your photography is bound to get better and you're bound to win!