It will just happen one day when you least expect it. After that, you should be able to figure out how to make them happen on a regular basis.
1. It has a lot to do with your mood... There are times when I know right off the bat that this time I won't be seeing stars...
2. It has a lot to do with finding exactly the right position that works for you. You'll feel it when you find it... It feels good... Move around and experiment.
3. It helps if you trust your guy 100% that he's not going to hurt you at all during sex. It's hard to come if you're gasping every time he enters because you're afraid he's gonna hit you 'there'...
4. Be sure you at least know what an orgasm feels like so you'll know it without a doubt when it hits. (i'm talkin' about auto-stimulation here, now)
good luck!
Ruby