teh SDK is teh base of the halflife code, you need it to write a new mod.
If you knwo nothing, I strongly suggest learning a little C++ (
http://cplus.about.com/library/blcplustut.htm )
Also, don't try and make somethign liek a mod when you don't knwo they core basics of how half life works, it's not a bad thign to want to learn but remember we're not here to do your dirty work, if you want to learn how to code, there's tutorials we coudl find you, but I sure as hell ain't going through all teh steps, it's a vastly complicated business to get it right, that's why there's a 1.6 version of CS, they didn't get it right first time, or in 1.2 1.3 1.4 1.5 etc, and those guys made HL... (well, the two guys that started cs didn't but they are now employed by valve so whatever)