Just some things to keep in mind:You're likely to get more hostility than help, especially from the cops (even if you're the victim, and there are guys about your age that like to jump the homeless for fun as they know the cops won't care, and it's not unknown for the cops to beat you for fun, too, sometimes trumping up charges on you after or even just leaving you there and claiming they never saw you before), but most people will be apathetic toward you. It's especially frustrating at election times as the mayors and what not urge the cops to lock up or drive away the homeless in "street sweeps" to make it appear something has been done about the homeless (and prostitutes, etc) to maximize their chances of reelection.
Never leave your stuff alone unless you got someone you trust to watch it. For example, a food bank I used to volunteer at allowed the homeless to wash clothes but once in a blue moon someone would steal the clothes or whatnot while the owner was out having a smoke break, and given the anonymity of the homeless as so many are forced to stay on the move it's easy for them to get away with it. That said, should you decide to be so despicable yourself then be aware that if you DO get caught by the homeless that they can take nasty forms of revenge (one guy they caught doing that had his hand broken with a hammer while others held him down).
Be sure nothing rattles like pills or coins in your pack or your chances of losing it get much higher.
Keep your bags on the side away from the road as they're less likely to be snatched by someone in a car. When in a new city be careful about looking at landmarks (that is, look where you're going, not all around like a tourist) as people who are new are seen as easy victims by certain predators.
I understand many (though not all) homeless shelters are run by little dictators (and that's not even counting how many have mandatory religious services) so be careful of that (plus, there are a lot of unstable people and addicts who come through and they can provide their own problems). Also, be wary of taking jobs here as some crooked people will hire the homeless and then rip them off hardcore as they know there's not anything the homeless can do (one guy told me he worked 12 hours hauling heavy metal and was then only paid $10 for the entire time, and he was too exhausted to beat the man to death as he wanted, but it was all part of a crooked operation run by a corrupt preacher).
Don't expect much from the welfare system, at least not in the States. It's a real pain to get anything, and what they do give really isn't enough to live on (and if they find out it IS, like you're getting free shelter, and continually staying at a homeless shelter counts, then they'll cut back on your bennies).
Frankly I'd camp, but cops, forest rangers, and the like really take a dim view of that (and some have returned to find their tents and gear confiscated or slashed to uselessness by the authorities). These are generally called "tramp camps." Though if it gets big enough (though this may only count in an actual urban area) then they might be called a "tent city." (Abandoned buildings, OTOH, are known as "squats.") There are plenty of guide books that can help you recognize what's edible and what's poisonous (be aware that many such plants are seasonal). A good water purifier (make sure it's good!) can serve you well here as well, and some do manage to live off the land (you won't likely find them in the cities).
To make friends with other homeless then offer to share alcohol, cigarettes, etc (illegal stuff also works). You should at least pretend to accept if you get offered (sometimes you can get away with saving a cigarette for "later" which you then use to give to someone else) as many don't take a refusal as "I don't smoke/drink" but rather "I won't smoke/drink with YOU." Don't ask others about their background or why they're homeless, that's considered rude (however, it's ok to share your back story).
Be aware that cigarettes (especially rolled, though even commercial cigarettes can be dipped in something that will drug you when smoked, though my understanding is that these cigarettes will be stained and can be "tasted") and drinks can be spiked with something, including tasteless date rape drugs, also used for robbing someone as well as raping them (though this is unusual, but a homeless teen victim of it is likely to be the victim of a sexual predator, and even males your age face a lot of danger of sexual abuse and exploitation on the streets, though it's even worse for females).
Once you seem to be accepted then it's good to ask where to go (squats, food banks, tramp camps, etc) and they'll probably know.
Oh, and a trick you can probably use, if you're in a city with Covenant House (called "C-House") then they probably send a van about who gives out free lunches. Claim to be a year younger and they should give you one (but never tell them where kids are squating or give them the impression that any charity is helping actual juveniles who are runaways as that can get the charity shut down for contributing to the delinquency of minors!).
Pizza places make for the best dumpster diving (the food is protected in a box), but these days it seems everyone puts locks on their dumpsters to prevent the homeless from hanging around to get at it. Oh be aware that some of the homeless can claim a dumpster as "theirs" and take exception if you get any food or recyclables out of it (but offering them smoke, booze, etc, can sometimes soften them up).
Once you're on the streets you'll find that it's very hard to get off as employers want stable addresses and phone numbers and the like. It's also real easy to get a criminal record, and not only will you find yourself in a position to be committing crimes just to get by but many cops don't hesitate to trump up charges on you, even plant evidence on you (it looks good on their reviews and some are sheer bullies who get off on the power of it, plain and simple). If you're lucky then Goodwill will hire you (and be sure to check out the thrift stores when buying stuff, including places like Goodwill).
Unless you're adverse to it I think you should consider the military if you're that desperate to get out (though of course that will provide a brand new set of challenges itself, but at least your future prospects are better). In which case pay very close attention to whatever agreement is made and read the fine print as recruiters lie (at least here in the States).