Check with your state's secretary of state or state corporations office. There should be a specific form for incorporating as a nonprofit. The 501c3 form, IRS form 1023, makes your group exempt from federal income taxes but doesn't affect corporate status, which is handled on the state level. The reason you incorporate first is that the IRS considers you a new organization once you incorporate. Thus, you would have to refile for 501c3 status if you incorporated later.
Also, you may need to file for income tax exemption with your state. It's a much easier process than filing with the IRS and typically requires information that you've already put together for form 1023. Some states automatically grant you state income tax exemption when you receive your 501c3 exemption from the IRS.
Looking for advice - we are starting a PTO locally for a K-4 elementary. We've read a lot of the suggested set up articles and they all point to incorporating first and then filing tax exempt status. What's the proper incorporating form to use? We were looking on legal zoom - and there are c corps, and s corps. Then someone mentioned that we should do a 501 c 3 - for non profit status and that essentially incorporates us. Is the 501 c 3 the form we need and then go to tax exempt?