Hi,
1) As said, you want the small STEAM program on C-drive (default).
2) Next you create a folder on the HDD, which you might call "E:\Steam2"
3) When you buy a game, it will ask you which FOLDER to install to. Choose the "E:\Steam2" folder (or name you made).
Other:
You can MOVE a game if you made a mistake->
a) backup game
b) DELETE local content for game
c) RESTORE game, but now choose the correct folder
d) DELETE backup (or keep if you add MODS in case you need to reinstall from scratch)