Hi and welcome
You might find
this a useful general guide, but the basics are:-
1. (most important) In the router wireless setup, make sure that encryption is enabled. For best security use WPA encryption - if there are several different flavours offered, then I suggest WPA-PSK as the most generally compatible option. Give it a good hard-to-guess pass phrase.
2. It may be worth changing the wireless channel to something other than the default value, just in case your neighbour is using the same router with the same default.
3. Change the SSID to something other than the default name. This isn't of itself a security issue, but if a cracker finds a system using the default name it may encourage them to have a go at cracking that system, on the assumption that it hasn't been set up in a professional manner.
4. In the PC, set up the dongle with the same security settings as the router, and that's just about it.