Look for the quick start leaflet which came with the router for details of IP address and router setup login username/password.
Best (but not essential) to start by connecting the computer to router by ethernet.
Put the router's IP address into the address bar of your browser. Login to the router setup screens and go to wireless security. Select WPA TKIP PSK as your security type (unless both router and wireless adapter support WPA2). Create a passphrase and write it down in the router documentation.
Save and exit router setup.
Go to wireless adapter's use interface and apply wireless security type and passphrase. Disconnect ethernet cable and see if you can now connect and browse the internet reliably.