There are hundreds of steps, no lift and the stairs are quite narrow so you have to be careful when passing people but the views from the top are amazing and well worth the 6 euros you pay to go up there.
There is a gift shop at the bottom selling mostly the same stuff that is sold in almost every musseum and tourist site in Florence and there several terraces part way up where you can have a sit down and rest, but if you can, you should try and climb to the top where there is an enclosed walkway around the top and you can see all around and get great views of the city.
Climbing this tower takes a lot of effort and it can be difficult for people with heart problems, disabilities or general health issues. There is no wheelchair access to the tower so if you cannot walk, there is probably little you can do here. Those unsteady on their feet may have some difficulties but as long as you take it steady and rest where you can, it should be managable. I am 28, overweight and have asthma and for me it was a struggle but I made it all the way to the top so for most people it should be easy enough.