Some of wannabe surfers don't need to exercise. They are naturally buff. For most, regular routines are a near necessity. Here are a few that will help you build the strength, balance and stamina you'll need out on the waves.
Upper body strength is a key element to controlled surfing. Uncontrolled surfing is called 'falling'. One good way to build it is cheap and effective: push-ups. The standard push-up (back and legs straight, legs together, hands a little less than shoulder width) builds biceps, triceps, deltoids and lats all at once. It's also good for general back strength and glute firmness.
You can gain upper body strength in other ways, like using barbells or a weight machine. But ordinary push-ups have some advantages. They cost nothing but your effort. You don't have to buy a set of weights or pay gym fees. If you need that to get motivated, though, by all means do so. Results are what count.
Push-ups also help you practice a movement that is a natural part of surfing - moving from a lying position to a standing crouch, called a 'pop-up'. At the end of every set, do the 'surfers jump' by pushing off as hard as possible, then drawing your knees up under your chest. Assume the traditional crouch position and hold for ten seconds while someone tries to push you over by nudging your shoulders.
Part of that upper body strength resides in more than just your arms and chest or back. It comes from the abdominals as well. Sit-ups are an inexpensive and effective way to build them up. Crunches are a shortened, higher effort form of a sit-up. Graduate to them when you can do more than 50 sit-ups in a minute.
Leg strength is a key element in surfing, obviously. Standing up doesn't take much strength. Crouching requires only modest effort. But standing up and crouching when a narrow piece of fiberglass-covered polyurethane is wiggling underneath your feet takes a lot of leg strength.
The reason is that you have to do more than stand or crouch, you have to maintain your balance. To do that takes good quadraceps and very toned calves, with strong buttocks. One great side benefit is that the more fit your leg muscles are, the better they can maintain your joints in a stable position. That reduces the risk of joint injury quite a bit. More than one surfer has had knee problems from being knocked off a board by a strong wave.
Overall stamina is equally vital. Paddling, maintaining a dynamically balanced stand, and other elements of surfing takes a lot of endurance. Add to that the need to swim to shore from time to time and you've upped the need fivefold.
Get fit, stay fit and workout regularly. Your surfing will benefit.