Firstly, make sure you're connected to the Internet. Also, configure your security program and Windows Firewall to allow Counter-Strike to communicate. After that, click Internet from the Find Servers menu and you should be able to see a list with Counter-Strike servers. If you want, you can use Google search engine to find servers where you can join. In order to connect to one of these servers, you will need to press the ~ key, to type connect and to enter the server address (IP:Port).