Much like Mike I found a site that looked good ( and could do UKARA numbers) booked a rental gun then turned up on the day at the time they said in my combats ( most don't need this they will include something in the rental normally but I had a set from my cadet days), a pair of hiking boots and the desire to have a good time.
Get stuck in, don't cower behind cover, the worst that will happen is you will have to walk back to the regen. On that note hit taking is important, again the worst is you have to walk so if you aren't sure take the walk. Don't expect to be amazing, rental guns aren't the best in the world get to understand the limitations of your gun, many around you will have spent a small fortune on their kit, it will probably outperform yours to begin with so adapt, get sneaky, its more fun anyway.
If you by lunchtime you are having a great time and thinking you want to continue then start having a look at the kit others are sporting, you probably were already but this time have a look at all of the different guns and stuff that is about. Many who have spent a small fortune on their kit are more that happy to talk at length about it, if you are friendly and happy to chat people will probably let you hold their baby some may even let you fire their guns.
If you are still keen and you haven't been put off by the small fortune you will likely be spending shortly make sure you get the UKARA process started before you leave, play 2 more games over a period of longer than 2 months but less than 12 with the same site operator and they should be able to get you sorted.
Have a laugh, don't take it too seriously, congratulate someone who gets the drop on you or gets you with a great shot. Enjoy yourself.
If you do most of that you'll be loving it, within my 3 games for UKARA I was invited to tag along with one of the local teams for the day at lunch I was swapped onto the other side to balance numbers and played against them, I think that must have made an impression I got a team patch and we now meet up regularly at lots of different skirmish sites.