You would install the software same as any other program.....
1) Chose which Free CAD Package you would like to use (if the one you want is not free you will have to purchase a license).
2) Download said software.
3) Install software.
5) Get to work.
if you're looking for ways to use autocad, you will have to be a student to use autocad for free. you will have to register at the autodesk site, provide proof that you're a student. you can also download the trial version and use it for free.
just in case... read the forum rules here: http://www.tomshardware.com/forum/38307-63-read-first