As far as figuring out how much water, etc. you have to guesstimate at first. Once you have your process tuned up, you can plug your efficiency numbers into beersmith (or any other software) and it will tell you how much grain etc. to use very accurately.
Until then, you can add water or extract to get the gravity you want, or boil down to get higher gravity if you want instead of adding extract (although you will lose volume that way too.)
Don't worry, I was worried about AG too, but this weekend I did my first and it well VERY well, even though my preparations consisted of running to the hardware and back to work (machine shop) to tweak my equipment.
The actual mashing etc. was NOT hard. Wish I'd spent a bit of money on going AG a long time ago!