If it hurts.

Try to change the position, ask your partner to move less intensively and enter less deeply. Does not help? Take a lube and use it - perhaps it's all about vaginal dryness and insufficient natural lubrication. If this has not helped, think about it: maybe you are very tense (especially if you have sex with this guy for the first time) and you just can’t relax. Pay more attention to the preludes to be fully tuned in to the “working moment”.