I haven't started a PTA but can tell you a few things from what I have seen at my kids' elementary school. This is in Chicago, so I assume it's a K-8 neighborhood school? We are in a Chicago burb, but low-income (Title I) school - the income level will drive your choice of activities to some degree. I think it's a very good idea to ask what has worked or not worked in the past.
Our school's PTA fills in the gaps of what the school can't cover, e.g., scholarships for field trips, bus rental for field trips, uniforms for sports, fundraising for big overnight for the 5th graders who can't afford the trip otherwise. The PTA also organizes events like a kindergarten graduation and 5th grade farewell (it's simple - we serve cake and watermelon to the families after the ceremony where the kids get their names called and sing a song), holds a welcome picnic in the fall, runs the school book store that sells used books for $0.25, manages the school garden, does events for teacher appreciation week, etc. The school events cannot cost too much or a lot of the families can't come, so we tend to have cheap, family-oriented events. Our fundraisers for the PTA are things like gift wrap sale (I hate that, personally!), an annual fundraiser with entertainment (high school jazz quartet-type stuff) and a silent auction of donated stuff in the school gym, and an iPad raffle (they got a donated one, so sold tix for $5).
Basically, you'll want to know the profile of the families at the school to make your activities as inclusive as possible. That includes whether to schedule events during the day or evening - you may want to switch off to accommodate parents working different shifts. Check whether you need an interpreter for events or translator for flyers. Our school houses a Spanish program, so at times both of those are needed.
Another challenge I have seen is people being super enthusiastic with new ideas but then falling down when it comes to the follow-through - usually in terms of having enough volunteers to carry out the ideas. Start small! Institutional memory is also important. Once someone has run an event or program many times, it's easy to rely on them, but you need to train someone else to take over well before that person's kids graduate.
Here is also the page I found for IL PTA: http://www.illinoispta.org/membership.html
Interestingly, our high school recently dissolved their PTA is going forward with a less formal group that is not officially affiliated with the administration, although the school holds the parent group's funds (like it holds the student club funds). This means the new parent group doesn't have to be incorporated. So, there are a couple of ways of doing it.
Anyway, best of luck and it would be interesting to hear what happens!