Well, I would start 2 ways. First, I would search " carnival" on these boards, and extract as much information as you can.
Then, set up a tentative plan as to what you want to accomplish, how much time you have ( 1/2 day, full day) and whether or not you want events that require volunteer supervision, or whether they are self sustaining " stations" that require little supervision. As far as costs, that depends entirely on what events you do. it can cost anywhere from $200 to $20,000. Will you have a band? Will you rent bounce houses? Clowns? prizes? food?
The list is virtually limitless.
Is this during the school day, or is it after school hours, where families are included? Doyou plan on charging? How much has your PTO allocated for the event? If it's during school hours, then you will have to work within a finite budget and plan accordingly. If you plan to charge for games, food, etc.. then what you charge will be a function of what you spend.
Once you spend some time thinking through some of these thing, and after doing research, come back and let us know some of your specific ideas.. and we can then guide you in a more specific direction! For example, how big is your school? Ages of kids, etc...
we look forward to helping you.. so come back with more questions