Detailed instructions for securing the network are detailed in the CD-ROM that came with your wireless device. Dig out that CD and take a look at the pdf files it contains on your PC. Brief details might be found on the booklet that came with the wireless device.
The wireless sevice is secured via the settings page that you open via a web browser. With my Belkin wireless modem/router, typing
http://192.168.2.1 into a browser window brings upthe page. Other manufacturers use different http addresses.