A reputable trekking company has a TAT license (with the TAT logo on it) that shows the company name together with a member number clearly written in English (the rest of the document will be in Thai). The company will need details of your passport to inform the police before the trek starts.
On the trek the company will provide one guide and at least one helper per group of 5-10 trekkers. The guide should be able to explain the customs of the hilltribes and will act as a transicitor. The guide will not just let you take care of yourself in the village unless you so wish.
A respectable trekking company is Buddy Tours