～ことにする
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.
くるまをかうことにしました。
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.
みいにちじゅういちじにねることにしています。
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