I can't seem to find it, even though it's probably looking me dead in the face. I'm new to Illustrator so I'm still trying to figure out the ins and outs of it. I'm using CS5. Any and all help is appreciated!
What AdamSchuld says below is the standard way (transparency window).
A second way of setting blending modes with finer control on what they apply to is via the super-useful
With this, you can set the blending mode (and opacity) of individual fills and strokes, as well as of certain effects. You can also give elements multiple fills and strokes with different blending modes to each other, which is sometimes useful if you're doing something complicated.