Nicole, you can use GIMP which is free software - it's like Adobe Photoshop. It takes some time to learn how to use it from what I hear, but it's really useful. For my graphics, I use Paint Shop Pro and/or Photoshop.
You will need to size the banner at exactly 760px by 100px so that it won't look blurry.
Also, there are lots of designers on Etsy who do that kind of thing. Just search "custom Etsy banner" and look for a designer who does something like what you have in mind.
It can save time and money to have someone do graphics for you. If you do have custom graphics done, ask the designer to give you the working files with all the layers and keep the files in a safe place in case you need them in future.
