im no expert, but if you bought your router recently, it probably came with some network security software.
the way to check it is by logging in to your router. You do this by going into your web browser and typing in the ip address of your router into the address bar. i tihnk you should be albe to find the ip address of the router in the manual. have a look at all the settings on the router and with a bit of luck there might be some security stuff.
jsut a bit of general info on wifi security: the most secure is MAC address filtering, this onyl allows the computers you specify to use your network. Next is WEP and WAP this are just different systems where you ave to enter a password, i think WEP is more secure than WAP, but im not 100%
And most important, good luck!