The primary job is to seperate airplanes in your airspace by using either radar, your eyes or pilot position reports depending on the situation. Also depending on the situation that seperation can vary from as little as 3 miles to as many as 20 (if the altitudes of the two aircraft in question are less than 1,000 feet apart). To achieve this you tell piloits to speed up, slow down, vector them (turn them), change their altitude or reroute them.
You are expected to do that in such a way as to minimise delays and in a way that will not cause any additional problems down the line. In addition to providing seperation in many positions you are expected to sequence all arriving traffic in a line so that they can land one after the other so that one airplane isn't still trying to exit the runway as the one behind it comes in to land. Of course, since you're doing this with minimal delays you can't have the second airplane landing 10 minutes after the first one exits the runway. The skys at the busy times are too crowded for that. Other controllers are responsible for all movement on the ground in an airport and some spend the day calling other ATC facilities for an airplane's clearance and relay that to the pilot. It isn't a one person does one task only job though. Without going into too much detail, every fully trained controller is capable of doing every position in their facility and might start out their day on ground control and finish up sequencing the string of arriving and departing aircraft on 4 seperate runways ensuring there is enough spacing toi prevent a collision.
Since everything a controller tells an airplane to do will have an impact on the next controller to handle the airplane, air traffic controllers are frequently tasked with calling other controllers and telling them what they are doing with an airplane. They might even call another controller asking to borrow some of their airspace while they vector an airplane around bad weather.
Additionally, in radar positions, most commands given to a pilot are entered into the computer so that the system knows what the plane is doing. The job does take a fair bit of multitasking ability as you'll find yourself writing or typing as you talk.
Best advice I can give is to say that most people live within an hour of a control tower. Find out which ones are near you and pick a less busy one. You can surely get their phone number from a flight school on the airport and just call and ask if you can come up and take a tour. Let them know you are thinking about ATC as a career. You won't be able to tour a very busy place like JFK or LAX, but touring a smaller facility and talking to the people there will tell you more than you can learn by someone typing on the internet or reading articles.
One last thing. DO NOT go in to aviation for the money. The new contract isn't paying what it used to. The median salary they quote in articles is higher than the new contract's highest salary. The money goes up and down in this business faster than any other business out there. I know pilots who borrowed 40 grand for their flight training just counting down the days till they got all of their ratings and started making 6 figure salaries and could pay off that debt in no time. They had a rude awakening when only regionals were hiring and they got their first flying gig making 17 thousand a year. Do it because you love the work, not the money.