The above is correct, despite the snarkiness. More detail
Go here and download the latest build of forge (currently 272). http://jenkins.minecraftforge.net:7070/job/forge/lastBuild/
The download link isn't well marked, so here's a direct linkhttp://files.minecraftforge.net/minecraftforge-universal-18.104.22.1682.zip
How to install it (for windows)
Go to Run (Windows key+r is the shortcut)
Type in %appdata%
Go to .minecraft (should be near the top)
Open it and go to bin
There should be a file there called minecraft.jar
Right click minecraft.jar and do 'open with...' winrar (if you don't have winrar, you need to install it - google)
Delete the file MATA-INF
Go to the file you downloaded, and copy everything in there over to the minecraft.jar folder
Close and you're done.
If you end up with any problems, it may be because you didn't download a fresh minecraft.jar. I skipped that part because it wasn't a problem for me.
To do this, go back to minecraft.jar that you just edited, and delete the minecraft.jar file. Then, open minecraft and connect, and it will download a new one. Then just repeat the steps above.