Having a server is one thing, making it active (not empty nor only one player) is another! If you look on the list of Counter-Strike server you're going to see that one third of these servers are empty. I don't think that you would want to be in between them...
Promoting a server isn't easy but you need to understand the rule of thumb that the more people you have playing on the server, the more people will come to the server because usually more people = more fun! If you have a lot of friends at your school or work who play Steam, that's great because you can ask them to come join your server all at once... You could even start off a great server with 10 people.
Another strategy that works very well is to post your server's IP address on your Steam related website... But not everyone has Steam related websites you know!
If you participate in the Steam Powered forums, you can put in your server's IP into your signature.
Last but not least. Remember to give your friends your external (WAN) IP address! In the internet world, you have two IP addresses, your internet and external address. You use your internal address for file sharing through your network or for connecting to your server from the same network. The external IP address in used to communicate with your computer from the outer world. If you run the command ipconfig in your command prompt, you're going to see your internal IP, to know your real IP address please use www.ip.gstutor.com. That site will show you your external IP address. Then give to your friends your external IP followed by an colon and your address. If you're IP address would be 126.96.36.199 then you should give 188.8.131.52:27015. If you use another port than 27015 then replace 27015 by your port.