I went alone my first game with a cheap trishot, a pocket full of shells and a well worn pair of dpm trousers. Suffice to say I was shown many times why even though shotguns are awesome you will be cut down more often than not haha
Tips for first game:
1. Hydration! You will be running around (hopefully) so you will get dehydrated. Take a large bottle of water with you and leave it in the safe zone.
2. Call your hits! It's a game of trust. If you cheat you'll be labelled as such and possibly banned from future events at that site. If in doubt, call yourself out.
3. Take what you need. Yeah it's fun to roll up in full combat gear like some dudes, but for a first game (with no UKARA) you will only need a rifle, some BBs, face protection and sturdy boots.
4. Rent your gun till you get UKARA sorted. Self explanatory really. You won't be allowed a shiny black toy till you've met the UKARA criteria.
5. Talk to people. Great ice breaker is to ask about their weapons - everyone likes to show off.
6. Read Ian's excellent guide:
http://www.airsoft-forums.co.uk/index.php/topic/17294-ians-guide-to-noobism-on-a-tight-budget/
Above all enjoy yourself and have fun
P.s. Welcome to the site mate