Just tell her, she'll be happier knowing than not knowing and wondering if she may've done something wrong or peculiar. Besides, if you're 21, you probably have a fairly small refractory period (you'll be able to get hard again fairly shortly after cumming). Just set aside a few hours for play, so you two can really genuinely enjoy each other, and it'll be fine. And she'll be able to mold you a little, teach you what precisely she enjoys without having to work through you possibly being stuck on what worked for otehr chicks (but may not work for her necessarily)
how bout if your in this same situation but with a girl you only just met...and its on like the 2nd date or even the 1st meeting. would you tell her that early? seems very hard to do in my opinion.
M8 just tell her its been a reely long time since u did it, then she'll expect u to be umm well bad, but u know, atleast she thinks uv don it. u could tell her the last girl u wer with broke ur heart, and so on u get where im goin. probably a bit shalow but it might get u by, just make sure u do some research so u got the basics down.