I always camp out there for the weekend so can't really help, but a friend usually drives in each day and parks in some private parking that's not too bad of a walk (within one mile of entrance at the main grandstands). I'm pretty sure some of us discussed it on the COTA Seems Like It Sucks thread, so you could look for some details there.
I love the weekend. I've been every year. Sure there are lots of challenges and issues. You have to go into it knowing it's going to be a complete clusterf and approach it with a sense of humor.
Comfortable shoes, light clothing, maybe some layers including a light jacket or windbreaker because it's going to be a little chilly in the mornings but it'll get hot throughout the day.
Each person can take in one factory-sealed 20 oz. water, and/or you can take an empty Camelbak to refill at the various water stations around the venue. You should definitely do this.
The first two days, I never bother sitting in my seats. I usually walk all around the track to see the cars from different angles and viewpoints. That's also a good time to scout out whatever shade there might be. There are usually some pretty large tents set up all around. They'll fill up on Saturday and Sunday, which are the busiest days. Friday is actually my favorite day out there, it's less crowded and less frantic.
Don't buy merch at the racetrack. It's crazy expensive and you can often find it cheaper online.
However, if you get caught up in the moment and feel you must buy merch at the racetrack, do it early on Friday. The lines at all the merchandise stands get much longer as the weekend rolls on.
Plan to buy as little food as possible at the racetrack because it's also crazy expensive. Eat a big breakfast before you go. You can take a backpack, and we often sneak in food like sammiches. Just wrap it up inside a windbreaker or something, inside your backpack. They rarely check bags all that closely. I've been known to sneak an entire charcuterie board including cheese knives into the venue.
Have a good attitude about it all, and have fun! It really is an extraordinary event. Good luck, we're all counting on you.