Every now and then (often enough to annoy me) my Airport decides to have a self-assigned IP address, and I have to restart my computer for my internet to work again..why does it do that?

Also, when I installed my wireless router I did not use a password for my network..I now want 1 considering I started to pick up other networks up around my home and I want NO ONE tapping into mine..how do you put a password on it?

thanks, DV