I'm new to designing websites..

Err, are you talking about making a design mockup in photoshop? Because the actual code you'll want to do in Dreamweaver.

 

960 x 560 is what I normally use as a baseline for 1024x768 displays.

You can create custom Dreamweaver templates to significantly reduce the amount of work you have to do for each page, and ensure consistency from page to page.
Basically, a Dreamweaver template has editable "regions" that change from page to page, such as the main text block, photos/graphics/features specific to a given page, etc...
The rest of the template is unchanged from page to page. (navigation, banners, logos, etc...)

Do a google search for dreamweaver template tutorials, or read the dreamweaver manual for complete instructions on using this feature.

 

You are familiar with PHP, HTML and CSS but do your Website completely in Photoshop? I, personally, don't think that's a good start. Why don't you try to design your first website with CSS only and after that you can add images using Photoshop.

 

I usually shoot for 900-1000px if you want to work with something that's easily dividable use 960px. You should slice where you need slices. If you have some practice with CSS you'll kind of have an idea of where you need to slice and how big you need images to be.

@ Epyfa: He's not talking about creating the entire website in Photoshop, only creating the design and then coding it out once he finds the look he's going for.