What is the make of your wireless modem?
Just for now, lets say it is a Belkin54G. This IP address is:192.168.2.1 but others are 192.168.1.1 or 0.1 just depends on the make.
Go into control panel and network connections, right click on local area connection and go into properties, next select Internet protocol (TC/IP) and select properties.Now select use the following IP address and Now set your IP address to 192.168.2.2 and subnet just click and it will put it in itself, next at the bottom Default gateway type in 192.168.2.1 click ok. Now turn off your router from the switch, wait 30seconds you should see your computer say network cable unplugged. and a red X in the little monitor bottom right of your screen on the bottom bar. This will disconnect anyone who is at this moment in time using your connection and also who may also be using the IP address of 192.168.2.2. Turn the router back on again and when you see the connection is bach up again. Go to the address bar on your IE browser and type in 192.168.2.1 and hit enter.
You should now get the routers splash screen or a small window open up asking you for the username and password. if it is set to factory default type in admin in both of them. This will let you into the router setup. Go to the wireless security section and set it to WPA/WPA2-Personal(PSK) and enable it. Set your password make sure it is something you will remember and that no-one else could easly guess. Don't use childrens names, dates of birth and the ilk. Use something obscure and remember the syntax of it, as it is syntax dependent in the WAP key. Now enable and save it to the router. And log-out of the router.
Now you will have to go back to control panel and network connections, Local area connection and reset your IP address to Obtain an IP Address Automatically. Now when you connect to your wireless WIFI it will tell you it is WPA encrypted and ask you for the password. Type in exactly the password you set on the router and bingo. WAP Security is now set-up and your network should be free of free-loaders. OK? Hope this helps and is not too long winded? Have a good one!