Above the player lists there's a flag. Choose a flag to pick your side. After that, you should be able to click on an airfield on the side you chose. From there you can choose the plane you want and the load-out for it. The "create" button under your plane choice on the right hand side of the screen will spawn you in.
After you've landed, crashed, died
, you'll need to click on an airfield again and use create to spawn again.