Well if you're talking about MAKING a banner, people usually use Photo-editing programs, such as Photoshop (most common and the best), Paint.Net (free), Paint (on every PC, I don't recommend this one though, too basic), Photoscape (free)...and more. u can Google 'Photo editing softwares' and you'll find a lot of free ones to download.
And if you're looking for making quality banners, u might wanna watch tutoring videos (can go to Youtube of just Google it), they'll toon u how to do specific things and obviously you're gonna have to fiddle with the program for a while if you're not experienced.
link is an example of Photoshop video tutorials ppl make.
and
link is a video teaching u how to create a simple banner using Photoshop.
Once again, 'Google' and 'Youtube' have basically everything!
Hope this helps! :D