How do I prepare for lifeguard training?

Well you aren't going to get into shape in a week so you either are or you're not. Most lifeguards are taught how to use their head first. It's not about trying to see if you can overpower someone. That will get both of you drowned. I'd start by looking at some CPR books to prepare. This can be done in a week and will be helpful when you start your classes. If you have quality instruction you can pass this. Have faith in yourself. It's no harder than anything you have had to learn. Let's hope you, or anyone, never has to practice what they learn.

I know that where I live you must swim a 300 and get a brick from the bottom of a pool (10 feet I think). Basically just swim some laps every day and you should be fine.

work on pulling weights out of the water holding your breath for longer periods of time and have some of your friends pretend like they're drowning (the whole flailing around kinda stuff) and "save" them those are probably things that you'll get testing on something like that

Good swimming skills and strong stamina, which can be only gained by practicing and training. Prepare for CPR and enjoy the training.

Start swimming laps to build endurance (200-300 yards at at time). Practice doing feet first surface dives to the bottom of the pool and picking up a brick or some object. Know your CPR ratios, that is just as hard and important as the swimming! If you are allowed, practice swimming with the lifeguard tube, I've seen some kids do some funny looking strokes on the tube. You should be doing front crawl head up (approach stroke) or breast stroke head up. Good luck and have fun, remember that the staff is there to help you succeed :)

oh man, that was the hardest thing i ever did in my life. good luck.