Assuming you mean for a story:
1) Start with the basics: name, age, appearance, occupation, etc.
2) Then flesh out their personality traits: what kind of person are they? How do they act around others? What are their quirks and mannerisms?
3) What motivates them? What drives them to do what they do?
4) What kind of conflict do they experience? Internal and external?
5) How do they grow and change over the course of the story?
There are of course many other aspects to consider when developing a character, but these are some basics to get you started.