just decided? I'm guessing that you've thought about it and didn't just wake up one morning and say "Hey why not the Navy". As for pointers, find an Officer Recuiter, he'll have any and all info that you need. The ASTB isn't the easiest thing in the world. Look at the study guides on this site, and check out the ARCO books. (you can get them from any book store).
Good luck
Ben