Be direct with her about your fears that she is becoming more involved with sexual behaviors than you want her to be at thirteen. If she wants to have a boyfriend, set up situations where she can invite the boy to your home. That way, you will know what is truth or fiction.
I suggest you keep track of the kinds of things you feel (or know) she is lying about. Determine whether these lies are aimed at helping her fit into a more experienced group or are manipulations to get her into less-supervised situations.
Trust is an important part of the middle-school years. Reacting now with consistency and understanding her needs can stop further problems from growing.