Though many PTO's decide to incorporate before filing for their 501c3, it's not a federal requirement. Incorporation is a state status, 501c3 is a federal status. The fee for incorporating varies from state to state, but seems to be consistently low. In our state, we paid $20 initially and $20 per year to renew our incorporation. The form consists of basically name, address, and bylaws (Articles of Incorporation).
The primary benefit of incorporating is to provide a level of legal protection for the organization's officers. Also, the IRS views the incorporation date as the date your organization was founded, no matter how old your group really is. That can simplify the 501c3 application process considerably.
The federal id number is free and can be gotten by calling the IRS or visiting their website
www.irs.gov. The form you need to fill out is called the SS-4. If you fill out the form first, then call, you can get the number over the phone.
There is a one-time fee ($150 or $500) to file for 501c3, but no annual renewal fee. As FM pointed out, the appication form (1023) and the annual "tax" return form (990/990-EZ) are a little daunting and some groups invest in professional assistance. But take heart, many PTOs have the expertise to complete the forms themselves -- you just may not know it yet.