。
You should mind your manners.
Literal
Etiquette-manners [in-に] be-careful [must].
礼儀作法 ('etiquette, manners, code of conduct') is a compound noun doubling 礼儀 ('manners') with 作法 ('proper way of doing things'). 気を付ける ('to be careful, mind') is a set verb phrase, takes に on what to watch for. ~なければいけない is the obligation construction (slightly more conversational than ~なければならない).