If possible, bring one or two reference photos. Show the sides and the front. Tell your barber what you like about them and whether your own hair sits differently. A photo is a starting point, not a guarantee of exactly the same result.
For a fade, mention whether you want it low around the ears or higher on the head. For the top, talk about how much length you want to keep rather than simply asking for a short cut. If you style your hair with product, explain how you normally wear it.
Mention any difficult areas, such as a crown that sticks up, a sensitive scalp or a hairline you prefer to keep natural. If you are getting a beard trim, say whether you are growing it out or maintaining a particular length.
Before the first cut, it is completely reasonable to confirm the plan. It is easier to take off a little more than to put hair back. Clear conversation makes the whole experience better.