Where to start
Whether you're stating up a character for D&D or adding a fresh face to a story, it's always important to start with the basics. Imagine you're describing this person to one of your friends and they're stuck in the middle of the crowd, so pointing at them doesn't help. One of the easiest places to start is the gender, in the wise words of Professor Oak, "Are you a boy or a girl?" Granted, sometimes your character's gender may not be as simple as boy or girl and understanding that may be vital to the plot...
Adversely, the age of the character may seem important to some, but unless it's vital to a plot point, an approximation or even the apparent age is good enough. Often times readers aren't as concerned with the character's exact to-the-minute age as they are with the appearance of their age.
Next are the more tangible aspects of the character. How tall are they? What body type are they? What race are they? How physically fit do they appear to be? Do they have any visible scars, markings or tattoos? How long is their hair and what color? Questions like these give your character a structure. If you can answer these (and possibly a few more) will give you a pretty good picture of your character. So, now that you know what your character looks like, where do we go from here?
Just as important as a character's appearance is their personality. The easiest place to start is what motivates your character? Just like in real life, most people are motivated by one thing or another to get them through the day. Most people would not agree to work a difficult and stressful job without expecting some sort of compensation, similarly your character would not be with the group they're with unless they had a reason. Like with your friend in a crowd, why would she/he be there? What's going on that would make her/him want to be in that crowd? Or equally as important what's keeping them from leaving that crowd if they would rather not be there? This motivation can carry over into D&D very easily. Your character is probably not traveling with the group on a whim (unless your character is particularly free-spirited or generous) but because they have a vested interest in the success of the group. This interest may or may not need to be known to the whole group or audience, but it should be known to you and should be the driving force behind your characters actions.
If you're anything like me you've probably taken a number of personality quizzes to see which of your favorite characters you are most like, but something you may not have considered is to do this for one of your characters. There are a number of great personality profiles out there: from the famous Myers-Briggs test to the Enneagram to even tarot cards.
The Myers-Briggs test features 16 distinct personality types from a combination of 8 different aspects. Extroversion (E) or Introversion (I), Sensing (S) or Intuition (N), Thinking (T) or Feeling (F), and Judging (J) or Perceiving (P). For a more in-depth understanding of the Myers-Briggs personality profile I recommend reading the Wikipedia page. There are a variety of sites which offer the test for you to take, but if you'd rather have an overview of the personality types here is a full explanation of each personality type.
Similarly the Enneagram offers 9 separate personality types:
1-The Reformer: one who is principled and self-disciplined,
2-The Helper: generous and people-pleasing,
3-The Achiever: adaptable and driven,
4-The Individualist: expressive and tempermental,
5- The Investigator: perceptive and innovative,
6- The Loyalist: engaging and suspicious,
7-The Enthusiast: spontaneous and versatile,
8- The Challenger: self-confident and decisive,
and 9- The Peacemaker: receptive and reassuring.
For more information about the personality types The Enneagram Institute explains each personality type in more detail and the interconnectedness of each type.
Don't forget that it is possible to combine some of the aspects from each to make a more well-rounded character. Of course, try to steer clear of conflicting personality traits if you can help it. Don't be afraid to be creative!
Where to go from Here
Now that we have a general framework for our character we are all but ready to take on the world! Just to recap I'll go over the important steps in creating an interesting character:
1- Body: What your character looks like - height, weight, age, gender, discerning features, ect.
2- Motivation: Why your character is here - this gives you room to work in their backstory and leads us to
3- Personality: Who they are on the inside - their personality profile from one of the quizzes or another source that gives them a defining attitude and patterns of thought.
Now all that's left is to make it happen. Good luck!