APR??!! I am confused please help!!?

Ok so I applied to a best buy card and I got approved but for program b and only $300. It says the APR for purchases is 24.9% which may vary. Other APRs 28.9% which may vary. There are a couple more things that I won't list at the moment but I will if it can help answer my question. My question is if I buy something with this card and I pay it right off in one month will I be charged the full 24.9% APR. I am assuming that the APR rate is annually so only if it took me a year I would be charged 24.9% interest. If I use this card and buy something for $300 I don't want to be charged 25% interest if I pay it off in one month. That's $75 right down the drain. Thanks for your time.