Hi i have never put amp and subs in my car, i was just wondering if people know how to use a two channel amp to run one sub? can i just run it or do i have to bridge it?

Depends on alot of different things amp size,sub size, is the amp bridgeable etc.If it is bridgeable, bridgeing would be the way to go if the amp was only going to be used for the sub, again depending on the size of the amp and sub.

If you tell us what the amp is (brand & model) then we should be able to tell you if it can be bridged

Reading the manual would be a good idea. Don't ever assume an amplifier will work in a certain way. The last thing we want is for you to start another thread asking why your amplifier isn't working...

It also depends on the sub - is it 2, 4 or 8ohm? single, dual (or more) voice coils?
Again, reading the manual or finding specs on-line using the precise model number is needed.
If you post up full details of each, I'm sure someone will be willing to find and interpret the specs for you.

The amp is likely to say on it if it's bridgeable, but it's important to know if it can handle the resistance of the sub in Ohms - if it's a 2 Ohm dual voice coil (DVC) then the sub's voice coils could be connected in either paralell for 1ohm or series for a 4 ohm load.

Bridging an amplifier is running it's two channels in parallel mode, ie: halving the resistance from the speaker. So the above scenario would present either a 0.5 Ohm or a 2 Ohm load to the Amp. Most amps that are bridgeable should be just fine at 2 Ohm, but only special ones will run at 0.5 Ohm without hitting thermal cut out or even possibly melting fataly.

Oh, as well as halving the resistance from the speaker, it's forcing the amp to double the power it's delivering - that's why too small a load will draw too much power and meltdown the amplifier.
Home stereo speakers are typically 8 ohm, while car audio are 4 Ohm.