Hi Marcus
I'll try and help you out.
1. If you're birthday is Monday and you want to shoot Monday, perhaps you can plead with your good lady to CHARGE THE BATTERY before she wraps up your camera.
2. On the 450D you have various automatic modes, picture modes, semi automatic modes and manual mode.
3. The little green square is a totally automatic mode. You can use this out of the box without knowing anything, but neither can you change anything. It'll even flip up the flash if it thinks it needs it - which can be really annoying.
4. "P" mode is a better version of the little green square. It doesn't flip up the flash automatically. It will always expose reasonably well and you get to move your finger wheel to pick which combination of aperture/shutter you wish. You can push the Av button whilst moving the finger wheel to make exposures slightly brighter or darker (EC).
5. The "picture" modes (landscape, portrait, sports) tailor things a little better but only allow you to shoot in jpeg only - you can't shoot in RAW. Once you get going you'll find this a real draw back. If you want to shoot in a picture mode at a zoo: SELECT SPORTS. It'll put it in Av, with a big apperture, a fast ISO, multishot, AI Servo autofocus mode...just what you want.
6. The semi-automatic modes really get where its at, but they probably take longer than a day to get to grips with. Tv lets you select the shutter speed with your finger wheel and you set the ISO with a menu. It then looks at how much light you've got and selects the right aperture. Av lets you select the Aperture with your finger wheel and picks the right shutter speed depending on the ISO you chose and the available light.
7. Manual mode is fabulous, but we can talk about that in another thread perhaps.
So I'd go for "P" and you'll get nice shoots. But "nice is way short of fabulous" as they say, so I'd go for SPORTS or Av. Perhaps the most sensible choice for Monday is SPORTS because it is your first day...but as quick as you can after that, read the manual and start to use the semi-automatic modes.
Have a lovely birthday Marcus - wish I was on a zoo shoot Monday
All the best
Steve