어야지

Have to; I'd better

Says something ought to happen. Pointed at yourself it is a resolution — 'I have to try the cheesy baked spaghetti', 'I was going to buy something delicious' — and pointed at someone else it is a telling-off for not doing what they should: you're supposed to go in like this, you really should follow the rules. Which one it is depends entirely on whose action is being talked about. All these lines are plain speech; 어야 해요 states the same obligation politely and without the attitude.

Register: casual, plain speech