The first place I'd look is your school district's website - chances are it will include a bit of information about how you initiate the process locally - which is important info. I'd also look at
www.wrightslaw.org - it's a great source of info. You can also look on the "yellow pages" link at the wrightslaw website to see if there are any parent advocate groups in our area/state. If there is one, I'd suggest a quick call to them - our local parent advocate was *extremely* helpful to me when advocating for accommodations/etc for our ds - they were able to quickly answer questions like you've asked here, and they knew all the *local* ins and outs, which can be very important!
The other thing to do is to be thinking through what you see as your ds' needs and gathering together all the resouces/data you have so that when you have a 504 meeting with the district - you'll be ready. One warning though - where we live the chances of getting anything together in the summer (before school starts up again) when you are starting from the beginning, are basically zero. Don't let that discourage you though! If you have your paperwork together and your formal request (in writing) in to the district before school staff is back at the beginning of the year, chances are you will get a meeting as quickly as possible

Good luck with it! I also answered this in a bit of a hurry, so if you have more specific questions, please feel free to ask!
polarbear