〜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