Please give some more information

-Regular or Classic MC?

-What operating system?

-What version of java?

-Does your computer suck?

-and so on

Are you using any mods? And has MC worked prior to this?

The mods may be what's causing your crash, Did you just install a mod before it crashing?

There is the problem. The mod messed up your minecraft.

You will have to uninstall them all

Edit:

I have heard This is a very nice alternative to messing with mods yourself. 

Rockyracoon, try using Magic Launcher, it allows you to mod your jar file without touching the jar. It will also allow you to test minecraft after adding each mod to see if it crashes you so you don't have to uninstall all your mods.