So, you are trying to access your home WiFi network?
It should be fairly simple. Tap on the status icons (the battery, clock, and such), tap Internet connection, tap on the network you want to access.

Or are you like me, stuck on regular 56k modem dialup?
If so, then you can't connect the N900 to the internet using it as of yet. If you have a data plan with your cell provider, you can connect using that on the device. Chances are that it'll be a bit slow, but still about the speed as dialup. In my experience, it's faster than dial up, and I end up using my phones as connection modems over bluetooth.