
Ok, you opened up a random NES cover into Photoshop, I use CS3, so if you are usingCS6 or CS2, processes may vary, but hopefully this sends you on the right path.

2nt: Useful for templates, Blue Guidelines! Open the ruler, shortcut in CS3 is "Ctrl R" Click on the ruler and drag into the image feild, a blue line should appear. I suggest showing what would be the spine with it.

Now create a new layer, Ignore my doodles.

covert the cover from a BG to a layer, Right click on the layer Background, and select convert BG to layer. This allows the layer to have transparency, and have other layers under it.

Creating a Gradient! Select the gradient too, then click on the image of the gradient on the top bar under "image" It is not pictured here, but clicking that should open this window. Edit the Gradient to your liking.

When you are happy with it, click and drag on your new layer and the gradient should appear like MAGIC!

Now for the stars! Select the brush tool, and fiddle with the settings.

Click randomly. Change size and occupancy also with some of them.

Now move your stary night below the NES cover layer, use the Magic Wand tool select the Black and then defeat it. Holding down shift will make your next click add to what was already selected, Alt will remove. Holding nothing will deselect what was selected and selet the new area. The icons on the top will do the same thing.

For the text, you could re type it or manually select each black part in Ps and Os and Ds and such, or you could select the text with the highlight tool, move it to a new layer and then create another layer underneath the text with Mode: Lighten or something. Then merge the two layers, the layer on top will inherit the bottom layers properties.
Also:
Image => Adjustments ... everything you need for fixing color and contrast and such
Layer => Layer Style ... You can find Drop Shadows and Bevels and such here
Filter ... Sharpen, Blur, and a bunch of filters in here
The best thing to do is just play around in it, try every option on something, see what happens.
I would help make some covers, but I have been very busy lately, and I have my own projects I'm working on, hopefully this tutorial will put you on the right path.