This is the perfect place for this sort of question!
Your mac is just remembering the preshared key to save you from entering it every time. If you want to change it, you can do it from the wireless network settings, but I don't know where they are on a mac, sorry.
Nobody can access your network without knowing the pre-shared key so it's about as secure as you can make a wireless network. To make it even more secure, you can choose to hide (or not broadcast) your networks SSID so that anybody looking for a wireless network will not see yours.
You should be able to set where you can access the routers network page from as well. It'll be under something like Remote Router Access or Management Control. Make sure it can only be controlled from the LAN (Local Area Network, your side of the router) or disable remote access.
All this stuff varies from router to router, so just access the config pages, click through and see what's where. While you're there, change the password from the default to something you'll remember but'll be difficult to guess.