Securing a wireless network

[azz0r]

I went war driving (correct) with my uncle and it was really cool but quite nerving, he just showed me how many unsecure networks there are about (we went through 5 posh neighbourhoods and found about 30 open wireless connections).

Anyway, Ive bought one for myself so my mom has the internet on the computer I made her downstairs, but now Im thinking how can I secure this network?

Its a d-link router and then a card in her pc, ideas?