if you're not doing the printing, then you shouldn't have to worry about separating the colors. The screen printer you work with will (should) do that.
As long as you have high resolution (200dpi at the size you want it printed) artwork, all you need to do is email that to the printer as a PSD file and they'll be able to take it from there and print your t-shirts.
If they need any changes, they'll be able to let you know. For example, if you created the design at a low dpi and then tried to resize it "up" in photoshop, that might make a pixellated image. You'd need to start you design at the high resolution blank canvas and design it from there.
If they are asking for a vector file, you can use a service like artworksource.com
and they can take your photoshop file and make it a vector file for usually under $20.