Keeping your surfboard flat underneath you while you surf can only be accomplished after you understand how to keep your weight centered in the middle of the board. Here are a few tips on how to get it done.
When you surf, you must stand sideways on the surfboard. With either the right foot near the back, which is called "regularfoot", or with the left foot towards the back, which is called "goofyfoot". Neither way is better than the other, but you will be better one way or the other. If you skateboard or snowboard you should surf the same way. If this is your first boardsport then stand on the board the same way that you stand in the batters box. Right handed batters go right foot back and left handed batters go goofyfoot. As a matter of fact the stance for surfing is very similar to the batting stance. Feet spread wider than shoulder width, knees bent enough to lower the body for more balance and the ability to adjust your weight. But instead of holding a bat, your hands should be held out away from the body for better balance.
It is the upper part of your body that needs to be most centered. It is critical to keep your hips centered over your feet. Then you can use weight shifts by applying more weight to one foot or the other to keep the board flat. The flatter the board stays the farther you will go on the wave.
When you land on the surfboard your front foot should be near the center and your back foot about 18 inches to two feet from the back. The wider apart you can spread them the better, as you will need a wide low stable stance in order to rise up and be in control of the board.
Surfing is fairly easy... once you are up in the stance, that is. So here goes... this is the hard part... the pop-up. When coming up to your feet after catching a wave you will need to grab your board at the rail. It is very important that you reach back to the middle of your chest, right at the bottom of your ribcage, just like when doing a push-up. If you do not grab the surfboard here you will have poor leverage and the pop-up will not be your friend. First, bring the shoulders up until the elbows lock, while leaving your knees on the back of the board. Now summon all of your strength, and using your stomach and leg muscles kick your hips up as high as they will go. Now bring your feet off the back of the surfboard and bring them underneath you and land in the middle of the board.
Both feet leave the board at the same time and both feet land back down at the same time. Be careful not to bring the back foot up too far or your stance will be too narrow. Once you begin to land in the middle the surfboard, it will remain flat so you can release your hands from the rails and rise. As you come up into the stance, continue looking at your surfboard as you may need to start shifting your weight right away. Now you are up and flying to the beach with the greatest of ease... surfing !!
Practice this technique in the shallows in the whitewater until you can come up and surf at least half of the time. Then you will be ready to paddle out and do the really cool wipeouts on the twenty footers outside.