if you went to this website http://files.minecraftforge.net/ it will have a 1.6.4 latest and a 1.6.4 recommended

i cant
find any files called forge 1.6.4

^ that (you can get to .minecraft by searching for %appdata% in your computer. Then a map named "roaming" should be the result and inside will be the .minecraft folder)

1. Download the version of forge that goes with the minecraft version you wish to play
I recommend downloading the installer if you dont know how to manually
http://files.minecraftforge.net/
2. run minecraft and click edit profile then select your version and choose the one that says forge(forge version here)
3. start minecraft with forge selected to let it load and generate the files needed
4. open your .minecraft folder
5. place any mods you wish to use in the mods folder
6. run minecraft again and if it doesnt crash it should show them in game and you can play


if you would like pictures showing what to do just ask

Single PLayer

For your server? or Client-wise?