Using a DISTINCT clause to filter data but still pull other fields that are not DISTINCT

I am trying to write a query in Postgresql that pulls a set of ordered data and filters it by a distinct field. I also need to pull several other fields from the same table row, but they need to be left out of the distinct evaluation. example:

  SELECT DISTINCT(user_id) user_id, 
         created_at 
    FROM creations 
ORDER BY created_at   
   LIMIT 20

I need the user_id to be DISTINCT, but don't care whether the created_at date is unique or not. Because the created_at date is being included in the evaluation, I am getting duplicate user_id in my result set.

Also, the data must be ordered by the date, so using DISTINCT ON is not an option here. It required that the DISTINCT ON field be the first field in the ORDER BY clause and that does not deliver the results that I seek.

How do I properly use the DISTINCT clause but limit its scope to only one field while still selecting other fields?