Oh that's easy, first of all what kind of character, for what kind of RP do you want to run? Then it's just a matter of filling out a skeleton, which will generically speaking look something like this
Name:
Age:
Gender:
Physical description:
Personality:
History:
but can also have all sorts of other fields like race, alliance, allegiance, superpowers, skills, things-in-your-pocket, grade (school), rank, etc, all depending on the sort of RP. Beyond that decent is relative, the people I play with wouldn't accept any character that included images in place of text descriptions, some people demand an image of your character, some people like to see several paragraphs for the last three fields.
If you don't have an RP in mind for the character I'd say go big, the more information you have to begin with the better you'll be able to make the character fit into any circumstance.
Also, I've found
several of these guides very useful in developing my characters