First, find an airsoft site to go to. There are many excellent directories out there, including the one provided by Airsoft Nation.
Next, book yourself in, usually by emailing the game organiser from their website.
It is highly recommended that you drag a friend or two along to split fuel costs (you might get lucky and your friend might get into airsoft as well). Don’t worry about bringing a big group of people like you often have to with paintball, in airsoft you will always team up with other players on site. If your group doesn’t have access to a car, then unfortunately it may be difficult to play airsoft long term as airsoft sites are usually in the middle of nowhere.