To regain your currency there are a few things you need to do legally.
Medical: The first thing you will need to do is get your medical cert back, to fly with a CFI this is not mandatory since they are technically acting as pilot in command but you will need it before you can act as pilot in command.
Bi-Annual: You will be required to do your bi-annual flight review which legally is one hour of ground instruction and one hour of flight time instruction to become current.
Landings: Although you will most likely knock this out during your bi-annual, before you carry passengers you will need to do 3 TO/LD for day currency and 3 full stop night TO/LD for night currency. Since you may need a few hours of instruction to feel comfortable I would set up at least 1 night hour to get these landings in.
In terms of what has changed in the last 30 years...
The 172 is still a 172 and the yoke is still in the same place. In terms of what you will encounter in the cockpit this will vary from plane to plane. Lots of trainers are old planes and or set up as they were more or less 30 years ago. The biggest difference you will find is the role and inclusion of GPS. While its not required (yet...) a lot of planes have the common Garmin 430 or 530 units and some planes out there may even have a full G1000 setup. Much of this depends where you rent from. You will still be able to get all your paper charts as you once could but many people are moving into the tablet/e-flight bag revolution which can really revolutionize how you fly. You may want to check out ForeFlight and the like to see what they have to offer.
The FAA offers lots of free handbooks that can be found here. Of particular interest might be Pilot's Handbook of Aeronautical Knowledge, Aeronautical Information Manual, and this overview on airspaces.