Depends on how bad it is, if its light then you can get away with something like 3m light oxidation remover or something similar lots on the market but stick with a good named one,same with if it is medium oxidation, or even heavy build up,also depends on the gelcoat as well, is it good and thick through or has it been done and worn down before, because every time you do it with a compound (which is what these things are) you take a minute layer everytime, so once you know this info then you can make a decision on which product to buy,I have used the 3m light stuff on my boat with great sucess, but the key here is to FOLLOW the instructions, and it takes a while, my boat is 36 ft do the math and double it and the best time is on a cool cloudy day, the product does not dry too fast and gives you time to WORK it, if its really bad then you will need a pro to COMPOUND it as it takes a skilled hand with a buffer who knows what they are doing,if you do it and dont know what you are doing then you can damage the boats gelcoat and never be happy with the job,I always wax mine when I can when I'm sitting on a dock as I travel the RIDEAU system often every year, and it gives me something to do with my time, also helps on resale value when the boat looks great, you can eat off my engines and engine bay they are so clean, everybodys so supprised mine is a 2004, godd luck hope this helps. :-)
Myles advice is good.
To get specific, you could post a picture and be more descriptive. Dark colors and more than 8 or ten years old, often means living with the chalking or painting the boat.