Either you want to be a parent or you don't - despite popular belief, babies don't just happen. I suggest using some kind of solid birth control and using it consistently, especially if you don't want to get pregnant. And I agree that you shouldn't be screwing around without protection if you can't even afford a home pregnancy test, especially since you can get them at the dollar store.
Also, I strongly advise against the shot. It might be the most convenient having to receive it only three times a year, but the side effects can be absolutely horrible. Some women feel no side effects, but others can feel things as crazy as panic attacks, hair loss, muscle soreness, and heart palpitations. The shot also is said to increase your risk of osteoporosis. Definitely let this be your last option, because there's so many horror stories about the shot out there that it'll make your head spin.
I suggest the iud - it's temporary, effective, and you don't need to remember a pill every day or a patch every week. It may be painful to have inserted if you have not given birth, but it will be so worth it when you know you're protected.