The 'correct' way to lay out a script for your comic book depends on what you're writing and who you're writing for. Find out what you need to bear in mind for the sake of you and your creative team.
I paid about £40 for a piece of formatting software called Superscript, which is specifically aimed at comic writers, but I wouldn't recommend it. I've bought it now, so I may as well use it, but it's very basic, even for a relatively cheap (compared to Final Draft etc) program.
Thanks for this Alec. I wonder if you would be kind enough to offer some of your thoughts on the difference between thought bubbles and inner monologue captions, and why the former has largely fallen out of favour. Personally, I don't use thought bubbles, but I notice John Wagner still does occasionally.