my xbox 360 broke

[MetalMilitia]

Not sure how much you know about this kind of thing so don't take the following post as being patronising:

Some really good thermal paste is Arctic Silver 5, but when you apply it do not apply too much; there should only be a very thin layer covering the entire core.
You should also ensure that the core is completly clean before applying a new layer of paste. There are specialised alcohol based cleaners available or if you have something similar it should work just fine.

If the base of the heatsink appears slighly rough or tarnished give it a good clean with fine wirewool (away from electonics!). It's not 100% neccacary but will give better thermal contact between the core and HSF.