It is quite simple, so be brave - you can do it.
1. Connect the laptop to the router by ethernet cable. This stops you getting locked out when you secure the wireless connection.
2. In the address bar at the top of the brower type
192.168.1.1
and press Enter (you type that in the big address bar, not the little search box)
3. Your admin router page should now open. Put your username and password in to the boxes. If you haven't set them, just type 'admin' in the username box and leave password blank.
4. Click the 'wireless' tab and 'wireless security'. Set the mode to WPA Personl; the algorithm to TKIP then put a strong password in the box. Do not forget the password.
5. Save settings.
That's it - done.
There are more settings / filters and so on you can do, but that is the most important. You do need to change the username and password too - do it in the Administration - management section.
Don't forget that if something goes wrong, at the back of the router is a small reset button you operate by pressing with a small point such as a pencil or pin for 10 seconds. This will take it back to the way it is now.