The problem is where to start. There are several different types of server out there. Listen, Dedicated and SRCDS.
SRCDS is the only one that will work when "your" PC is off because it generally doesn't run on your PC, it runs on a server in a datacentre somewhere or, assuming you dont live in the UK, on a high speed connection to your house.
Now. If you want to run SRCDS follow Xtreme's instructions on how to download SRCDS and your already on your way.
As for mods and things like that, thats what these forums are for. Search around for people with similar posts that may answer your questions.
Be warned tho. You arent very specific in what you are asking people to help with. I agree with Xtreme when he says "add pictures?". People cant help you unless they know what they need to be answering.
Good luck.