Well, let's start at the beginning. Is the airport card installed in your iBook G4?
If it is, then you're basically ready to go. Read the system help files about "airport" to learn the step-by-step of turning the card on and finding a wifi network.
If you do not already have a wireless network set up at home, you will need to buy a wireless router. Despite the extra expense, I strongly recommend an Apple router -- they are just WAY friendlier, more reliable, work with PCs just fine, etc.
Using wifi outside your home will depend on whether the places you go have wifi or not. A lot of places do, but most of them are protected networks so you have to have their password to use their network. Some coffee shops and other such businesses offer free wifi, and its usually pretty easy to sign in and log on. Most of the businesses that offer this have a sign on their door mentioning it. You'll quickly develop a short list of favourite hangout spots that offer free wifi so you can just enjoy your iBook and have a drink or a meal.