A quick brief:
Have your panel layout on on layer, with your artwork on a layer below, and a layer on top where you can create your speech bubbles and the text in them. Online tutorials should help you out. If you're having too much trouble, you can try Paint.NET, which is like Photoshop but simpler.
http://www.getpaint.net/
There are also some programs you can use that can help simplify the process. Manga Studio is a popular choice for black and white comics. However, the Pro version is pretty pricey.
http://graphics.smithmicro.com/go/mangastudio
There's also TokyoPop's Manga Creator:
It's much much cheaper than Manga Studio, and easier to use. The dowside is that it has less features. It's up to you what you'd want to spring for.
http://www.animenewsnetwork.com/review/tokyopop-manga-creator-2
There's also Comic Book Creator 2, which is fairly cheap. Which usually means, easy to use but low on advanced features:
http://www.myplanetwide.com/products/detail/description.html?id=32
It's up to you to find out exactly what these programs can do and decide whether it's worth paying for that extra functionality.
If you're on a Mac, Comic Life should have come with your computer. There's also a PC version.
http://plasq.com/comiclife-win
If you know how to use bittorrent, most of these programs can be found cracked online.