Make then believable
There should have emotions, goals, fears, strengths and weakness. Avoid making your character too perfect. Everyone has a good and bad side.
Describe your characters.
When describing your characters avoid putting a long description. For example. She was tall, blue eyes, brown hair, curvy hips, big lips, big smile, long legs. Try to throw their description around. She tied her long brown hair in a ponytail and started jogging. Give your characters distinctive features or habits, a scar on her arm, a bird tattoo on her necks, rubs hairs when nervous.
Having a leading character.
Who is your leading male or female character. Avoid having too many leading characters. The leading character is the one who is there from beginning to end.
Don't have too many characters. When editing your story make sure the character has a purpose in your story and if they don't, chop them. One character can do more then one thing.
Give your character names that you can pronounce
Don't give your characters names you cannot pronounce. Xenhyophino, if you cannot say it, don't use it. Stick to traditional names. If you are having trouble naming your character combine two people's name or use a phone directory. Same names will come naturally to you.
Give your character a backstory. Everyone has a past, where they grow up, what factors in their life made them what they are now, past relationships, parents, siblings. Not all backstories have to have a tortured past.
Friends and Foes.
Give your characters friends, everyone has a social life or someone they share their life with. Give them enemies, why don't they like a particular person, cause conflict between your characters, it adds the intensity and drama of your story.