I'm afraid we can't offer technique advice here, firstly because it's illegal at a site that has to be appropriate for 13 year olds, and secondly because there is no magical technique that guarantees orgasm.
However, there is still some advice we can offer.
Firstly, are you comfortable when your girlfriend is performing manual sex on you? Being comfortable and relaxed is important when engaging in any sexual activity, and it is definitely harder to achieve orgasm if you are not. If you aren't comfortable, then it's totally okay to not engage in that particular sexual activity until you do feel ready for it.
Secondly, even if you are comfortable, it could be the stress of wanting to reach orgasm preventing you from actually having one. Orgasm doesn't have to be the goal of sex, and if it feels nice, then don't worry too much about whether or not you're going to have one. It will probably happen eventually, and in the meantime, you may as well enjoy it anyway.