To create a white spot color, open the Swatches palette (Window > Swatches if it's not visible), double-click the white swatch, and change it from process color to spot color. Create your white base with this swatch color (in other words, create the shape, select the Fill color, then click the swatch). If you create white another way (such as 0% black) it may not separate.
If you are using a full color logo along with this white base, you will have five separations (Cyan, Magenta, Yellow, Black and White). Start by going to File > Separation Setup... and click on Open PPD... The easiest thing to do here is to use one of the PPDs supplied with Illustrator, so navigate to the Illustrator folder, look for Utilities > Sample PPDs and choose one of the files. Click OK.
To make the separations, go to File > Print... and look for a drop down list that says Output: Composite (it's exact location depends on your OS). Change this setting to Seperate, and Print. That's assuming you need to print the separations yourself - if you're sending them to a printer, they'll probably do all this for you.
