Okay first few games, play it safe. See how people play, if they do something right and are doing well try and learn from them and learn from your mistakes. Don't spray and pray, use mainly semi and not much full auto..
Then, when you get a bit more confident start pushing up, trying different things if all doesn't go according to plan. DO what you know, don't try and do a tarzan monkey swing across the trees, its never going to happen
Enjoy yourself, I'm sure you will but don't be too competitive, its good to be competitive to a certain degree and then when you get too serious it doesn't become fun anymore.
Take plenty of water and snacks, and plenty of BBs, depending on what battery, you may need to take, (probably not if your above 1000mah though)
ALWAYS TAKE YOUR HITS, people will respect you more, won't whine and you will have a better day, saves you being told off as well.
Try and talk to people, good first line is "I like the gun mate, what is it?" or something along those lines. People love to show off and talk about their gun. It's how I got some of my mates through airsoft.
Don't be afraid to tell a marshal if you think/know someone is cheating, tell the marshal / or alternatively light 'em up (not really advised )
If in doubt, don't do it in terms of rules and what you think you can do.
Finally, find your comfort zone then start exploring around that, don't sit at the back of the site and get out there and get some exercise and have some fun