People can't just
tell you exactly how to make good banners. =\
You need to actually know something about the program you're working with, so you know what does this and that, how this can edit that, ect.
Having a 'good banner', is kind of a matter of opinion. Some people might absolutley adore a banner, while other people hate it, just personal preference.
Firstly, though, you need a good graphics program. I use Photoshop CS 8 myself, and I suggest you get Photoshop 7 or CS (or Photoshop Elements, I haven't used it but I heard that it's almost exactly the same as normal Photoshop, just a cheaper alternative o.O).
After you have that, go to
https://www.good-tutorials.com .
It's a great place to start out, and you should easily be able to find something that appeals to you.
If you need help with certian tools or fucntions of Photoshop, feel free to say so and I'll help you out as best as I can.