"Lettering" is adding the word balloons and sound effects on top of comic book art. I letter all my comics myself. It’s actually a minor art form. I think lettering is best when it's "invisible", when the reader doesn't notice it at all. And invisibility is a lot harder to achieve than it sounds.
Most comics are lettered on the computer these days– it's a lot easier than doing it by hand. If look closely at the word balloons, you'll see that all the "As" (or whatever) are exactly the same. There are lots and lots of computer fonts out there made to mimic hand-drawn comics lettering.
One free piece of advice is this: please, whatever you do, don't use the font Comic Sans. I know, It's called "Comic Sans". You're creating a comic. It's already on your computer. It's so tempting! But don't do it, for the love of God. Just don't.
So where to find good fonts? Check out Nate Piekos's blambot.com for some great freeware lettering fonts. If you're looking to "upgrade" and spend a little cash, he's got some great pay fonts, too. Another excellent site is Comicraft's comicbookfonts.com.
You'll probably use a program called Adobe Illustrator to letter your comics. That's what I use. Illustrator is an amazing piece of software; technically, it's called a "vector graphics editor", but that's a topic for another post. It's expensive, and it's worth it. You to learn the program inside and out.
I love the Comicraft guys. The mark of a true professional is someone who’s willing to share their secrets. No one is more generous than Comicraft it comes to spreading the wealth. You can learn just about everything there is to learn about lettering from balloontales.com.