You can draw from your experience, even if you don't think you've experienced what you're trying to write about. It can be something similar - similar in how you feel, not in the end result. One example I have read is how an author wrote about a girl who desperately wanted a nose job, because she thought that her nose was too big and it was preventing her from making friends. After the nose job, she found it made no difference. The author in this example had never had a nose job; what she drew on was her memories of having glasses - specifically, she desperately wanted contacts because she thought the glasses were making it hard for her to make friends. "Four-eyes" teasing and all that. When the author finally got contacts, she found (surprise!) that it made no difference.

Draw an analogy between your characters and yourself - what matters is the feeling. Strong desire, either in the acquisitive sense or the sexual sense; loss; satisfaction. Even if the thing or event that prompted the feeling is different, the feelings are related.

A little bit harder is writing a character whose attitudes are something you do not share - for example, a thief, when you've never stolen anything in your life. How would you get his thoughts and attitudes believable? A murderer? Someone with a very slim grasp on reality? Or even just an average person brought up in a different culture?

You can find reams of advice, lists of questions to ask about your character and his background, and those provide a great starting point. (Just remember: generic questions lead to generic characters.) But the essential spark that brings the character alive and makes him believable can not be taught by some simple formula, and this is why writing is both an art and a craft.

To bring the example back to the thief character: even if you've never stolen anything, was there ever a time when you showed a complete lack of concern for somebody else's property? Or to the murderer - say they've killed somebody to get them out of the way. Have you ever been so focussed on some achievement that you hurt (emotionally or mentally, perhaps) someone? You didn't kill them, but the person you hurt was a distraction, or was in your way, and you didn't care that you hurt them. What was going through your head at the time? Did you even realise what you were doing at that moment, or was it later?

What are some other types of parallels you can draw from your life?