There are a lot of fursuiting communities online (http://fursuit.livejournal.com
is one) that specialize in mascot-style animal costumes, and I'm sure they'd have some solutions for you here, too.
If the idea of associating yourself with furries is distasteful, well then here's a rundown.
A lot of people make their animal heads by building the base out of plastic canvas (usually used to make children's yarn embroidery projects, you can find it at a lot of craft stores for 25 cents a sheet or so) and building on top of it with upholstery foam to get the shape.
How big do you want to make this head? If you want to shrink it to fairly human-sized then this method might be your best bet. If you want it to be that huge, then maybe start the base by paper-macheing a beach ball...?
One thing to make note of is that a head like this is going to be HOT. Consider leaving spots on the lower back of the head (under the afro) open for ventilation, and be suuuure to have some sort of vent in the front of the mask so you can breathe.
That's my advice on getting started with the basics, at least.