If you need the answers to those questions, I'd suggest not trying to make mocks for the mocks forum - you'll get slated. Play around in photoshop*, find out what the tools do, use google to look at the thousands of tutorials on how to do certain effects and make things for your own benefit.
Once you've learned how to use the tools, start thinking about what you want to design (you could even use paper for this), use the tips, hints & templates in the stickies in the mocks forum.
Basically what I'm trying to say is, that if you don't know how to use the software you're planing to use
you will make a bad job of it
and
you will be unceremoniously slated
by everyone in there. Oh, and before you hit the submit post button on your first mock, just ask yourself "can I see this being used on TV?". If no, hit back and start afresh.
*other design software is available. I personally prefer Fireworks.