OK - the important thing here is to take it slowly. I know that might be frustrating but it will be worth it in the end.
I'm assuming if you couldn't get your cock inside her that the length is not the issue (how could be?) but that you have a girthy cock (?).
There are two main factors, lubrication and how turned on the woman is. And of course the more turned on she is the more natural lube she will be producing - but there is no harm adding some more. If you are using condoms then read up on what is a safe lube for condoms (something water-based) and please check with her to see does she have allergies - the last thing she needs is a swollen twat.
Even more important than that is that she be very, very turned on BEFORE you try penetration. Lots of foreplay - whatever she likes - kissing, oral, fingering, nipple play - whatever gets her squirming and wet. Then try again - really slowly and carefully and make sure she knows you'll stop if and when she needs you to. And then, if you do get in take it easy til she's begging you to fuck harder, and even then, don't go all out on her, not the first time.
If you don't manage it the first time then don't worry and don't get cross with yourself or her. Just ask nicely for a blowjob and try again another time - it'll happen :smile: