Form a committee. It would be impractical to try to run a fundraiser on your own. You need the help of other parents, so ask for volunteers. If you don't get enough volunteers, draft some.
Bounce ideas off each other. The committee needs to come up with practical ideas of what they want to do in order to raise money. The fundraiser could be a carnival, a school play, an auction, a car wash or even as simple as selling candy bars or a bake sale. A lot depends on how much money you need to raise.
Find out the amount of money you will need to do the fundraiser. Assign this task to one of the committee members. Of course, it will be better if you can get donations--which probably won't be a problem if it is a bake sale or an auction.
Decide on a date. After figuring out the type of fundraiser you will be having you need to pick a date for it. Make sure it gives you enough time to plan everything that needs to be done before that date. Even a bake sale takes some time to organize because you have to get commitments from people on what they will contribute.
Advertise the event in a school letter or whatever form the school uses to notify parents about events and other important things. If the school is affiliated with a church you can also use the church bulletin to advertise the event.
Collect donations and make sure everything is in place for the fundraiser. Assign different people to different tasks in order to make sure everything runs smoothly. If you are selling candy bars make sure you have people around each day to collect the money and give the students more candy to sell. Incentives to the students will help them sell more candy.
Pay your expenses and add up the profit you have made. This is all there is to run a school fundraiser. You can make it look so easy that you will probably be asked to do it again.