How to replace bathroom supply line?
I've used channel-locks and crescent wrenches, but a basin wrench will save you some busted knuckles.  I think that your 1/2" supply is probably coming out of the wall and is grey Polybutylene pipe?  If so, you need a 1/2" PB x 1/4" PB shutoff valve, or an adapter to convert to PEX and then the angle shutoff valve.

If you do indeed have a 1/2" supply to your old faucet (very odd indeed -- are you sure that it isn't just the connecting nut that is 1/2"?), then you can probably replace those with braided supplies of the proper size on each end fairly easily.  If you post pics of what is going on we can give you more specific advice.