I did my first tourney last weekend, and felt completely like you did before I went. FWIW, here's what it was like for me:
- A big part of being nervous for me was not knowing what level of intensity to expect - I had heard that it was a totally different level...was this going to be like a really intense training roll, or was I going to find myself in something like a street fight with BJJ rules? Answer: it was pretty much like a really intense training roll plus just a little more. Once the match got started, it felt pretty comfortable...so you can relax a bit on that score. Even if you're overmatched technically (and I was), it's not so different from rolling with a blue belt in training...being overmatched isn't the most fun thing in the world, but it's not going to be a 5 minute miserable nightmare either.
- First match, I got the "adrenaline dump," big time. I am normally fairly relaxed when I roll and have pretty decent cardio, so I can go 3 full-speed 5 min sparring rounds with no problem in training. In the tournament, 3 minutes in and I was sucking wind. Next match was much better, however. I don't know what you could do to prevent that except try to be patient.
- As soon as the match started, it was like my brain said "see ya later, guy." Ability to think tactically was close to zero. So it helps a lot to have someone to coach you on the sidelines and to be used to listening to coaching when you roll.
- I also competed in the masters white belt open and wound up in my first match against a pretty huge guy (like, 6-3, 240-ish) who I am told is a judo BB. Did I have a shot in hell of winning? Not really. But I still think it was useful in terms of getting comfortable with the tournament situation. I got tapped by that guy, but it was no big deal...and now I think I'll feel more comfortable in just about any future tourney situation...I mean, it's unlikely that I'll fight anyone more challenging than that.
- In terms of what to expect results-wise...how long have you been training? BJJ is mostly a skills game, and at the white belt level the differences can be pretty acute. In my case, I have only been training three months and I am certainly no prodigy, so I got schooled just about as bad as you would expect. But I think the experience of competing was useful anyway. Come next year I'll probably still be a white belt, but with 15 months of training I'll expect better results. If you're a high whilte belt, you'll probably do fine. If you're a low white belt, you'll probably get dominated. No surprise there.
- I did have a great day in class on Monday, I felt really sharp on the mat. So there's probably something to be said for competing even if you get beat.
Hope this helps. Good luck in your competition.