Make sure your and you partner do a lot of foreplay first. Women need to be fully aroused and wet before you have sex. If that’s not enough, use some lube.
So I'm not a guy, and I don't know how accurate this would be - but they make toys that "replicate" vaginas. You could maybe practice on those.
But that said, even men with loads of experience fumble around trying to enter a woman. It's not like porn where it just always works smoothly and no one misses or rolls the condom down the wrong way.
Make sure she's plenty lubricated - foreplay helps, and sometimes extra lube is needed, even if she's really turned on. Lots of things factor into this - alcohol tends to inhibit lubrication, some medications do too, including birth control, where she is in her cycle, etc. It doesn't mean she isn't aroused by you.
Ask her for help, too. She knows where it is, and that can help in the dark. Or turn the lights on.
Practice with condoms at home. Figure out what brands and size you like. And even when you do that, you'll still fumble sometimes in the heat of the moment. It's all normal, and happens to everyone. :)
Is this more than one girl you're having the issue with? In general, women stretch to accommodate. Make sure to use lube as that might help and try to relax (as should she). Takes time but you'll get the hang of all of this.