I think these are some very important points. Many companies are really just in the business of selling you a "volunteer experience". You'd be better off donating a portion of your money and spending the rest on a holiday. Same effect.
I'm with what Fyrefly and dyskontent say.
It's doubtful what you could accomplish in a week or two. More importantly, if its manual labor (or any labor), you are taking jobs away from the local population which is already impoverished enough. I can assure you these countries have no shortage of labor, it's more a shortage of work.
The industry they've set up is a bit dodgy, sometimes even stocking fake orphanages (google it, it really happens). I can understand the desire to help, especially the bright idealism of someone just coming out of highschool but the truth is, you as an 18 year old who (probably) doesn't speak the local language and has a few to none useful skills can't really help all that much. Even if its just teaching English or something like that, even then your ability to help is a bit doubtful.
I third the idea of donating a chunk of money to a worthy organisation, it's much more likely to help the locals and create some sustainable change. There are charities out there for *everything* and some innovative projects like
http://www.zidisha.org . I'm sure you can find something. Donate a chunk of money and use the rest to go on holiday, perhaps the same country you would have volunteered in (tourism creates jobs and helps the local economy).
Helping out locally is a great idea as well! Massive respect to begin with for wanting to create a better world than the one we're currently in though, if only everyone had the same ideal .