DI-514 - Wireless 2.4GHz (802.11b) Router

I have a couple of pcs hardwired to the router and a couple of wireless laptops which also share the connection, rarely more than three machines at any one time.

The settings on the router are currently unsecured and I would like to change the settings so my computers can all use it, but not someone nearby outside. I no longer have the manual for the router and any assistance to convert it to a secure network is appreciated, thank you.