I downloaded a PSD template from templatemonster.com
How do make the big PSD file a web page?

Just open the file in Photoshop and chose the "Save for Web" option under File. It will create an HTML page and save the images in a separate folder.

you will need to slice it if it isnt already

Good slicing can be a little tricky. (as I have found to my cost!).

This might be of some help. http://www.photoshopcafe.com/tutorials/

There are many other good tutorials out there that may help.

Great links, that will help me out as well.