There are a few basics you should know about before you even begin making sig sets.

Size
I always recommend using 100px height because most forums will have a 100x100 avatar limit and it is really easy to make an avatar by simply dragging layers from your sig into a new document. This saves you time! I personally stick to sig widths between 220px and 450px.

Colour
I usually choose colours by eye but if you don't trust yourself you can always use the eyedropper tool and use colours from your stock picture. That way the colours you use will definitely match because it will be the same colour!

Style
Sometimes simple is always best. A background, stock picture, main text and border are the bare minimum you need to make a decent sig. Don't go overboard and stick lots of text and pictures in because just like a room a sig is no good cluttered!

Stock
Your stock picture (the picture you use in your set) should be a decent size - nothing too small. You should never have to resize BIGGER because you lose quality! Never choose a picture that looks choppy because you won't be able to fix it. Bigger is almost always better in this case.

Fonts
Pick something that will stand out if your picture is boring, or something plain if your picture is extravagant. Don't pick something hard to read and always pick a decent font size because otherwise you will ALWAYS get the comment, "I can't read what your sig says it is too small/hard to read".