Hi, Search for tutorials and guides. [link] Your file should probably be at least 300 dpi at comic book size. [link] Start with something simple like a 3 page black and white story. There are standard practices for fonts and balloons. [link]
You are literally thinking about the least important parts of doing a comic.
Experiment, and see what looks good with the art you make for it. You shouldn't choose a font before you have sample pages to test it on; you shouldn't choose a size before you have a sample page to see if it looks good at that size; you shouldn't choose... well, you get it.