If you're planning on charity work, the only actual preparation you need is to make sure you have enough to go around. Master Legend is one of many RLSH who do charity work; he has a pretty good system, and is pretty well-known in his area.
Any work out on the streets requires a good working knowledge of at least...
-Basic 1st Aide,
-Leadership skills,
-Conflict resolution skills, and
-Deductive and inductive reasoning skills.
You also have to be careful, especially if you walk into a crime-scene for some reason, to not screw with any part of it, especially if the cops haven't gotten to it yet. It seems like one of those "No duh" guidelines, but it can't be stressed enough. Even with the best intentions, you go bumbling through an area where a guy has just been stabbed to death, you're liable to destroy evidence that could lead to his killer. First aide, of course, takes slight precedence, as you're keeping the victim alive, but avoid screwing up the evidence as much as possible.
It's always good to have some contacts in the area, who can provide information as to what kind of need there is, but it's not an absolute necessity. If you go into an area without any contacts, and you play your cards right, you might just meet some new people, and get some fresh information.