sigtutorials is a top site for tuts on sig making from beginner tuts and progressng as you go.
Using a stock is quite tricky … especially when learning imo
planetrenders is a fantastic site for renders ( cut out pics from stocks)
if you look at the stickies in this very thread you will see a simple tut from chilli on some basic stuff using renders.
dont think your sig or art has to be the size of the pic you have either … open a new page, choose the dimensions you want in pixels (ie i think mine is 450x200 … nips looks like 450x150 or so).
Then drag your image onto it (make sure you have the cursor tool/grab tool or what ever the fuck its called selected) . then it puts it on a new layer … then goto edit > transform > scale …it highlights the full image … then just drag your pic till you find a corner, then hold shift and drag the corner into itself and the image will shrink by scale.