Take the student aside, away from the others, and confront her in a calm, quiet manner. Do not call her a liar or accuse her of lying.
State what you have observed that led you to believe that the student is lying. For example, say, "When Joe was asking if anyone had seen his new crayons, you didn't say anything, but I saw you put a package of new crayons in your desk earlier. The right thing for you to do is to give Joe's crayons back to him and apologize for taking them, don't you agree?"
Redirect the student if she tries to lie about the crayons. Say "We all make mistakes. It's time that you correct yours, please."
Observe the student and be aware of any additional lying. Try to find the reason for his behavior. Perhaps he wants attention. If this is the case, provide him with better ways to receive it such as asking him to help you with some classroom chores.
Ask the student to repeat what she said if she tells you a lie directly. For instance, if you see the student talking and you tell her to stop, only to have her say " I wasn't talking," you can say "Would you repeat what you just said?" with raised eyebrows. This may discourage her from engaging in the behavior.