What do I need to start?
A simple knitting pattern (there are plenty of free patterns online)
I recommend buying the needle sizes either side of the recommended size as your knitting may be tighter or more loose than average. You don't want to start out and have to go out and buy more. Keep the package and reciept and you might be able to take the ones you don't need back.
Something to store your work - a knitting bag is handy and easy to make if you are sewing-minded or a green bag will still be fine.
Buy your wool all in one hit and check the dye-lots are the same so that you have the exact same colour each time.
Why do I need to buy wool from the same dye-lot? There may be subtle differences between different dye-lots. Very rarely can you find a big enough ball of wool for a whole jumper.