orgasm???

I recently started succuessfully masturbating but I don't know if I am reaching orgasm. I am a 16 year old virgin, and I have been single all my life. I only do clitoral stimulation. It takes me a while to actually become stimulated, but once it starts to feel good it lasts for like 1 minute and then I stop. I stop when my muscles start to shake and it feels like I physically cannot continue becomes it's too intense. Am I reaching orgasm?? If yes, how can I/ should i last longer for when I want to have sex. If no, how can I?