なくちゃ / なきゃ

なくちゃ and なきゃ are the casual "have to", "gotta". Take the ない-form of a verb and replace ない with either one: いかない行かない → いかなくちゃ行かなくちゃ, いかなきゃ行かなきゃ. You say them to friends and family, and very often to yourself.

なくちゃ is short for なくては and なきゃ for なければ. The two mean the same, and you can use either.

With いけない or ならない

The ending of the full form can stay on. It is still casual, but sounds a little more definite.

Register

Both are spoken and casual. With people you speak politely to, use なければいけません or なくてはいけません.

"If you don't…"

When the sentence goes on to say what happens, なきゃ and なくちゃ are plain conditionals: "if you don't".