You can do an ATP on a Cessna 172 but then you will only have an ATP Single Engine.
I suggest you get a copy of a FAR/AIM and FAR/AIM FC (flight crew which has 121 regulations).
You can take an exam for either 135 or 121 but you will get the same license. People say the 121 exam is easier. Really, the only ones who take the 135 exam are people who are more familiar with the 135 regulations, but when you pass the knowledge test and practical test, your ATP will be valid for both 135 and 121, except that the single engine won't be too useful. You would probably be better off doing the ATP test on a twin engine, such as a Seneca or Duchess.
There are some other things you will have to do but I don't have time to explain them at the moment.