I am boy of 18 years old .what i should to do that i will finished my sex time equal to my girlfriend, other mean how can i improve my sex time?

You are not alone: Average time for men (18-35) to reach climax is about 6+mins, and for female is over 15min. So, for most men, we play catch-up. Wearing thicker condom will lengthen this time. Talk to a sex therapist--for ideas to do it on your own, or consult doc for some off-label treatment meds such as prozac/zoloft/paxil and/or topical anesthetics may be of much help.