What type of person is your character. Is she a sunny optimistic? A hypochondriac? A manic depressive? A bully? Passive aggressive? Snob? Knowing your character's personality will help in finding ways to describe her to your reader. Sometimes, having a strong personality trait in your character will go a long way in describing your character to your reader, because your reader will also have preconceived notions of what a character of that type will look like.
Depending on what type of person your character is, ask yourself what that character will look like. As I wrote before, people have preconceived notions of that particular personality type. But what if you want to go against the grain? For instance, what if your shy wallflower actually looks like a beauty queen? Or maybe your ladies' man is actually a chubby nerd?
Make a list of your character's physical descriptions. Is your character tall or short? Is he thin or overweight? Does he have acne or perfect skin? What is the color of his hair, eyes, skin? Does he have big hands or small? Does she have a distinguishing beauty mark? Physical deformities? Try to visualize your character and write down what you think she should look like based on what you already know about her personality and how it will affect the story.
Make a list of your character's other attributes. What does he sound like? How does he talk? Is he inarticulate or erudite? How does he walk or sit? What does he look or sound like when he is happy or angry? Does he tend to blush when he's embarrassed? Ask yourself questions about your character. Try to get as many attributes as you can about him. You might not even use all these details in your story, but you'll have a better sense of who your character is.
What are your character's attitudes toward life? Is he happy-go-lucky or is he cynical? What about his religious background? Is he Protestant, Catholic, Baptist, or is he an atheist? How does he feel about work? About politics? About love and marriage? These details will also help you in shaping your characters' descriptions.
Describe your character's tastes. What type of clothes or shoes does she prefer to wear? What type of music does she listen to? What type of furniture is in her home? Does she like to cook and what type of cooking? French, Italian, gourmet? In Bobbie Ann Mason's short story, Shiloh, for instance, her character Norma Jean cooks a potato and mushroom casserole topped with grated cheese. This little detail opens up a complete world about her characters' socio-economic status, beliefs and attitudes.