ことにする means "decide to do ...." It follows the short form present tense of a verb. You can use a negated verb, too.
We have decided to buy a car.
ことにしている means "do something as a regular practice," that is, you have made up your mind that you should do something and have stuck to that determination.
I make it a rule to go to bed at eleven every night.
〜ことにする = decide to do ...
〜ことにしている = as a regular practice