Knowing what type of skating you want to do is essential in getting the correct skate. If you want to do a little of both, tell the salesman and he can help you out. If at all possible you will want to try on your skates and skate around the block. Try and figure out what feel good and where the problem points are. Remember, if the skate doesn't fit well after you are in it for 5 minutes in the store, they sure as heck aren't going to feel very nice after you have been on them an hour.
Many stores offer skates for rent.
If your perspective skate is available, use it. Frequently they also apply rental charges toward purchases so you have nothing to lose. Skate parts are also an essential part of a purchase. Metal bearing spacers are also very important for speed and stability. The wheels that generally come with most skates are crummy, so don't stress too much about them.