If you are a lady (それを）しなくちゃいけないの　Or (それを）やらなくちゃいけないの If you are a guy （それを）しなくちゃいけないんだ　Or （それを）やらなくちゃいけないんだ Instead of の、んだ,you can use んです to make a polite sentense. For example, （それを）しなくちゃいけないんですOr （それを）やらなくちゃいけないんです （それを）isn't obligation to say it, but it depends on the situation you are in. Hope it helped you. Enjoy studying japanese! 　

Actually there are lots of ways to say that in Japanese. Learning the whole list in one day won't help you. You need to learn them one by one. Be patient, man. You'll get there in the end.

やらなくちゃ。 やらなければならない。

私はあれをしないといけない。