In another post I read, it said to make an erection last longer or makin your penis a little bit bigger, when masterbating, stop right before you ejaculate, how do I know exactly when to stop and when it's too early to stop and start up again?
I'm not sure that it's gonna make your penis bigger! It will delay the orgasm/ejaculation and usually make it more powerful with more release of semen. You're the only one that can tell when it's time to stop, but it's gotta be before you feel it rising through your penis. Also, NEVER stop an ejaculation while it is happening (by closing the tip of the penis) this can be dangerous! You're at an exciting part of development....use the time to figure out your body and how it works. Take time to experiment with masturbation and different techniques, you may be surprised at what you can learn about what works best for you!