〜ないと・〜なきゃ
have to, must (casual)
Verb ない-form + と · ない-form minus い + きゃ
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Nobody says なければなりません between friends. Casual speech cuts obligation down to the condition alone, and the "or it won't do" part is understood.
ないと is short for ないといけない: もう帰らないと, "I've got to go home now".
なきゃ is short for なければ (ならない): 宿題しなきゃ, "I have to do my homework". Build it from the ない-form: 帰らない → 帰らなきゃ, 食べない → 食べなきゃ.
Both are extremely common, often with a small sigh: 早く寝ないと…, "I really should go to bed".
The full ないといけない is fine in polite speech too: 行かないといけません. なきゃ stays casual.
Register: Casual: the everyday way to say "I have to".
Examples
もうこんな時間。帰らないと。
Look at the time. I've got to go home.
明日テストだから、勉強しなきゃ。
There's a test tomorrow, so I have to study.
薬を飲まないといけませんよ。
You have to take your medicine.
早く寝なきゃ。明日は六時起きだ。
I need to get to bed. I'm up at six tomorrow.
あ、母に電話しないと。
Oh, I need to call my mum.