Bicycle Mechanics - Crank

I just bought a Giant Warp DS2, and after it started creaking, I tightened the crank, as someone from the store suggested. Well now the right side of the crank has loosened, and tightening it doesn't help. I am roughly 300 miles from the original bike store. What should I do and what needs to be replaced?

I just bought a Giant Warp DS2, and after it started creaking, I tightened the crank, as someone from the store suggested. Well now the right side of the crank has loosened, and tightening it doesn't help. I am roughly 300 miles from the original bike store. What should I do and what needs to be replaced?
If if won't stay tight it usually means the crank arm is buggered. The whole business may not have been properly tightened from the beginning and if that were the case,the store ought to eat it.

Did I void the warranty by tightening it though?

Tightening the crank should not void your warranty. If it is creaking and tightening it does not work ,take it to the shop that you bought it from and have them fix it. They should do it for free, the customer is always right.

The problem is that I'm 300 miles from the bike shop, but I am going back next week (I get a 4 day vacation). Can I still ride it as long as I keep the crank tight? I'm 15 and it's my only transportation (well, I have a BMX bike, but I'm just too tall for it now).

If the crank is just creaking and not coming loose on its own you should be ok to ride it as long as it is torqued properly , usually about 32 ft lbs. If it gets loose after being torqued ,the crank is shot ,like sydney said. Then you probably should not ride it. Good luck with it.