Dont play in your backyard with an airsoft gun, period.
For the rest, read a lot or go meet folks to talk to them in person.
You can contact people through the Teams section, or by attending listed games as an observer.
And yes, it's expensive. Think about it like it's driving a car; you dont take a tricycle on the highway.
The guns you want are so awful that you can probably throw a BB harder, and further, with more accuracy, simply with your hand.
This game requires good equipment. If you want to plink in your backyard, it's the wrong game/tools. And I think it's also pretty unwise to have anything that even looks like a gun in a city backyard (unless you really want to attract the cops).