Best source of timings is bahn.com
You’ll see there are S-Bahn trains every 20 mins (journey time 15 mins) with an extra regional train (journey time 14 mins) once an hour.
S-Bahn trains depart Cologne Airport at 04, 24, 44 mins.
The regional train departs at 08 mins past the hour.
All arrive at Koeln Hauptbahnhof (Cologne Central Station).