First, take off all the doors, drawers and counter tops. Many times, counter tops are glued to your base cabinets, and are bolted together at the seams with pocket bolts. Try to take these off carefully if you plan on re-using them, or donating them too.
Then, look for a board near the top of the base cabinets, this is the board usually used to screw the cabinets to the wall. Take the screws out that are being used to secure them to the wall, and it's likely that they'll can just be pulled away from the wall. Be careful though, sometimes, the base cabinets are screwed to each other too, so look at the insides of the base cabinets to see.
The same basic process can be done for the upper cabinets as well, except that you should make sure that they are not screwed to each other before trying to take one down. This is almost always a two-man job.
As for donations, I suggest looking for a Re-Store in your area. They are operated by Habitat For Humanity, and I personally have purchased many things (including cabinets) from them. All the proceeds from the sales of donated items goes to help low-income families earn the pride of home ownership!