I love flying and people-watching at airports. But if you're not used to it, definitely ask for assistance.
Keep your luggage to a minimum. You're not leaving the planet!!*. If you're going to stay with friends or family you will be able to wash clothes. Also, most American hotels have washing machines and dryers, so just take enough for 5-6 days.
You don't need a whole suitcase full of stuff on the flight, but do pop a spare pair of pants, socks, and your toothbrush, face cream and all medications in your handbag. Soap, shampoo and towels will be provided wherever you are staying.
Make sure you have good advice regarding how to pack liquids, rechargeables, etc., and how to fill in the ESTA application. Possibly from the travel agent or from frequent flyer friends, neighbours or relatives. (Apologies if you've already done all this.
If your luggage doesn't arrive with you, make sure you report it immediately and get a reference number to quote for further enquiries.
Don't let the immigration staff in Chicago intimidate you. I have never had this problem but I have heard some of them can be draconian . Getting through immigration can take anything from 10 minutes to 3 hours. So go to the loo as soon as you land!
And don't be scared to ask people to help.you around Heathrow before you are picked up. That would daunt even me. (Where possible, I avoid the big airports). O'Hare is probably huge as well, but getting out is usually easier as there is only one way to go.
Have a safe journey!
*having said that, if you were leaving the planet, I have heard that the personal luggage limit is 180 grammes!!