We'll need to know a lot more about your computer and about the issue. "Lag" is a very unspecific term, you'll have to give a more accurate description of the symptoms and when they occur, as well as thinking about some of the following questions, and possibly providing them here. What kind of specs does your computer have? What kind of internet, and what sort of network? What other games do you play other than Source games that you can compare this with? What kind of FPS do you get? Do you have any background processes running? Are you using a torrent program, Skype, or running some kind of server from your computer? Do you have someone else in your home or on your local network doing any of the above? Do you use wireless internet?
There's a lot of people here who are more experienced than myself as far as some of the source-specific problems go, such as console tweaks you may have to apply if you have an abnormal setup or weird settings, but you'd probably need to provide some of the above information, maybe a screenshot with your net graph up.