There's a fine line between trying too hard and just being prepared. So before your first date the best advice we can give is to prepare yourself by taking some time out for yourself.
Whatever it takes to get you nice and relaxed - do it. Whether that's a jog round the park, a de-brief with your friends or a little pampering, whatever it is to keep you cool.
If your mind is clear from stress or if you're able to stop over-thinking about possible situations for your date then you'll act much better on it - simple.