How to make a query for fetching a single record?

Hi all,
There are tables

categories
id
name
--------------
items
id
category_id
name

So each category can be up to N number of item
I do not understand how to join to put a limit
Here's the essence
I need to choose for example where in (categories_ids) from the items table one record for each category, and to be more precise for the latter.
Who faced? or anyone knows a solution?
Thank you