The size of your teils cage can not be to large. If you have to get a smaller cage get one that is longer in length then it is in height. Also make sure that the bars on the cage are small enough so that a cockatiel cannot get it's head through the bars.
Items that you will need in the cage are a feed and water dish, perches, cuttle bone and some chew toys. You will also need a nesting box if you are thinking about breeding your tiels.
In my own cage I have a water and food dish, a cuttle bone, 3 perches the lenght of the cage, a small perch that has two bells and rawhide on it, a ladder and an ear of corn.
Cleaning your tiels cage.
You will need to take everything out of the cage and clean it at least once a week. This includes washing the perches and the toys. The only exception is that the food dishes, they should be cleaned every day to help prevent bacteria growth.
The above picture is what a normal teils cage will look like.