also, your router would have come with instructions, giving an address to access the router settings.
it'll be something like "goto your web browser, type in 192.168.1.1".. but some manufacturers choose a slightly different address. go into that (it may ask for password, the default will be given in the manual), and goto the wireless page. there you can name your network, and also enable encryption. if your computers all support it, choose WPA (or WPA-PSK it may be called), and enter a password. note the "text" password that you entered, and the weird "hex" one that it may have generated for you. then just enter those into your computers that you want to give access to.