If you want to participate in certain classes/events you have to pass a float test.
We just took ours at Green Lake for free with a lifeguard there. You just need an American Red Cross certified life guard, on a beach or at a pool. If you go with the pool you have to pay the admission fee, however, so we opted for the beach.
After nine minutes of treading water you have to put on a lifejacket (while still in the water) and stay in for one more minute. I got mine on without difficulty but Alex had a hard time getting his second arm in, giving me and the lifeguard some comic relief.
You have to wear long sleeve shirt and pants and bring your ID. We also brought our life jackets, a copy of the float test form, and towels.
I recommend wearing tighter sports clothes to give you less drag while in the water. Ten minutes of treading water is not very exciting and seems to stretch on forever.
If you are going to the beach maybe go in the morning. We were able to get a lifeguard almost immediately because no one was in the water. They stay with you (ours was on a standup paddle board) for the whole test.