Printing images at 20cm x 30cm

I have a university assignment that requires me to print my photographs at 8x12 or 20x30 and frame them, by using a printing machine at a camera store. However, I have completely forgotten how to do this in Photoshop CS4, could anyone help?

If you are using a DSLR, you don't need to crop them to a different aspect ratio, because your sensor is already at the required 2:3 aspect ratio for an 8x12 or 20x30 print -- they made it easy on you!

So, the only thing you need to do is print the photo! For this you don't need any post-processing -- if you have properly framed/composed your photo and gotten a good exposure, you can do some tweaks in post processing or not, but the idea is it should be "good to go" for a photography class.

Yeah, i couldn't figure it out myself, earlier today, so i took a chance and took them to the store as they were. They came out quite nicely, and thank you for your comment