First of all you'll have to break down your photoshop site like you would a normal html site. It will go together just like that except that instead of actually putting in a lot of the elements directly, you will just put in the appropriate php codes.

A Decent Tutorial for making a webpage from Photoshop

Next, Joomla itself has a great tutorial on how to make a template. The real "how-to" doesn't start until about page 5 of the tutorial though.

Joomla Template Tutorial

I am not sure what medium you're using to move from Photoshop to Joomla, but if you are using Dreamweaver, download this extension to help in the process of making the template (and the whole site):

Dreamweaver Extension to use Joomla "tags".

Some good info you posted. Thanks

@ Kyle

Links provided by you are expired. If anybody knows about joomla template design please share it.

That joomla tutorial link is bad. Here is a different one that contains a very simple tutorial.

http://docs.joomla.org/Tutorial:Creating_a_basic_Joomla!_template

have a peek here: http://psd2cssonline.com/

It requires some pre-submission setup in photoshop, and I've no idea of the quality of the output. But even if it's not right, it will get you a template you can start editing from, taking care of most of the tedious parts.