You need to ask yourself what you want to do! Are you prepared to volunteer or do you need to earn at least SOMETHING? Would you want to go with a religious-based organisation or something completely different? Do you want to work with kids in the USA, or would you be happier building houses in Uganda?
There are a massive number of things...and yeah I guess you need to figure out what there IS before you can decide what you want to do, but now that it's April I think you really need to start applying to things. I know for Camp America it'll be nearing the latest date, as you need to get an American J-1 Visa (which is a bit of a nuisance) AND a Crimial Record Disclosure!
The reason that you have to be 18 for most of it will be all due to getting Visas for certain places...it's a bugger for some people I know!
(Also, if you want some advice...do a Camp America thing - go with BUNAC or CCUSA - best thing I ever did!)