1) which linux is the best?
Which ever one works for you. I recommend Redhat. Sticking with the package manager will help you down the road. If you need custom builds, you build custom RPM's. Don't be a tool and blow over everything with source, it will only come back to bite your butt when the package manager can't find your deps.
2) what would I have to do to be abel to choose which OS i want when I turn my computer on
I would use grub (grand unified bootloader) Place it in your MBR, it can load windows or linux.
http://google.com/linux and search for dual boot etc etc,
3) What (if anything) else would I need to know when installing it?
2 tips,
one, install windows first your first time, and make a boot floppy in linux!
two, google is your new best friend, use it.