Most forums have some sort of option to put your own images etc like you see here (top left), all you need to do is make the image the right size. If the forum dosnt give you the option to change it, you could just replace the image in the folder with your own, or you can alter the header template.
To make the image you just need to know the right size and then do a bit of cutting and editing in photoshop. See the design tutorials for specific design ideas etc, or even the banner req thread might be able to help you, although not sure if its for another forum
As for a skin, thats a little harder and it depends on the forum software your using, for stuff like IPB you set the colours to use for each bar and section and make a skin that way, or you can import 1.
If you have specific questions on the banner your trying to make we can give you a better answer on how to make it, but this section is meant for art/design type of tutorial requests, not for how to setup the technical side of a forum