The car wash is one of the most common and most recognizable fundraising ideas you'll ever come across. It's so popular because it works so well.
Identify a suitable location for the car wash, remembering that you need a reliable source of water to take advantage of. Schools, gas stations and churches on busy roads are all places that are well worth considering.
Your group will need to supply everything for the car wash: a garden hose, buckets, sponges, soap and towels. Settle on a suitable date as well as a backup date in case the first one is rained off (without a plan B you'd lose your chance to make money).
You can advertise in advance through flyers, local newspapers and on your city's cable station. On the day itself make signs for members of your group to hold to attract passing traffic.
It's up to you whether you want to accept donations or charge a set amount for your car washing services (you may make more by asking each individual customer to decide). Have some small bills ready in case you need to give out change. Enjoy the experience too — this will be reflected in your proceeds.
You can read our in-depth article on hosting a car wash here.