First let’s make sure your expectations are realistic. Many young men ejaculate within a minute or two of getting an erection. As you get older, the time between the start of erection and ejaculation will get longer. In the meantime, you should wear a condom (which you should do anyway to protect yourself from pregnancy and infections) which will delay the sensation and hopefully the ejaculation. You could also try masturbating before sex, as the time from erection to ejaculation may be longer the second time.