You should wait until you have enough money for everything you will need before you consider getting your pet. This hobby isn't cheap, but you will end up spending more if you don't make the right decisions the first time.
You can get very cheap used fish tanks on craigslist--sometimes people sell entire 10 gallon tank setups for around $20 or if you get really lucky, you might even find one for free. Just be very patient, it's difficult when you're young and you have all of your impulses, but no way to satisfy them. Just use the time between now and when you have more money to research exactly what you want so that you get all the right things the first time. Once you get your betta he will be with you for possibly the next 7 years--a little waiting now is nothing compared to that. Just be patient. :)