If you want to buy new surfboard, first you must check out a lot of things, like your weight and height and of course your skill level. Here you can see chart for beginners and intermediate surfers.
If you are beginner you should follow this guide and this will help you to maximise your standing up time. If you choose wrong surfboard you will fall a lot. This board types have right balance of your weight and floatation and believe me, you want to stick that together. You must buy perfect board or you will give up and throw out your surfboard and whole surfing thing.
But, if you are intermediate surfer, you know how to surf. But, I say but again because you also must check out this Intermediate Surfboard Chart because, like beginners, you also need a perfect board for surfing. You need semi-fish or shortboard, and these board will give you maximum. Check out this chart for board sizes.