Im 21 yrs old, male, and I have had sex with different girls. But I couldn't seem to get an erection. But when I masterbate by myself its not hard for me to get an erection. And I want to have an erection with da ladies but can't seem to. What do I do.

YOu need some foreplay. The girl needs to do something to help turn you on. Don't expect to have a full blown hard on with all of the girlys that slip out of their skirts. If they expect it to be like that then you need to tell that ho " Get off me, Biach." Please practice safe sex with a condom. I mean by putting on the condom not having sex with the condom itself. Let us know anything you need.

Hi,
Relaxing is the key. It could be that you are too "tense" and maybe worried a little bit too much about making sex really, really good.
How much time do you spend with a girl before you actually do it?
For example, do you lie in bed with her, for maybe 15 minutes, holding her, kissing, fondling each other?
This will come in time for you, and just don't "rush it". As another said, it's called Foreplay. She can have you hard in a few mintues, if you just relax.
I do wish you the best of luck.