Alright I assume this MS Visual is linked to coding?
I mean to make a mod you need levels right? So you use the SDK found in Steam and a program within that called Hammer which you use to make levels for the Source engine. If you want to make things like custom weapons then you need to model them in a modelling program such as XSI, 3DS Max, Maya etc. You'll then need to export it and rig it and then code it in.
So I mean if you have no skills at all in any of these sections then I highly suggest you start with Hammer in the SDK (Source Development Kit) in your Steam Games List. If you need tutorials on level design then I suggest a few places but since I have Hammer / HL2DM open at the moment I am only going to link once since long lag is looooooong.
http://www.interlopers.net/index.php?page=tutorialsHave a look at these and I hope they help. Feel free to ask anything else.