First of all, you have to stop worrying about it. Trying to make an orgasm happen makes it less likely to happen (I am not sure that is helpful advice, but it is true). Hopefully you enjoy sexual activity even without having an orgasm.
Second, it is much easier to have an orgasm with a caring partner that you trust. First of all, sexuality is about having a connection with another person, that's how we are designed. Second, if you are with someone you respect and care about, you can relax and enjoy being with them until you have an orgasm. It takes the pressure off.
You might have luck with either erotic fiction, or pornography (many women do). Still the key is to get your mind off of having an orgasm so you can relax and enjoy the erotic experience.