Take it to a bike shop.

You would need a crank extractor tool jsut to remove it, then you would have to selct a new crank set that would fit you bottom bracket for sa correct chainline, yada yada. Not something a novice wants to tackle solo.

See...

http://www.parktool.com/blog/repair-help/categories/crank-service

to see what's involved and the tools you need.