From Amsterdam Centraal, Almere Centrum is a 22 minute train ride on an Intercity train. It is about 40 minutes on a Sprinter train. The trains cost the same amount, so try to plan to take the faster INTERCITY train.
Trains that go to ALMERE CENTRUM, LELYSTAD, ZWOLLE, GRONGIGEN will all most likely stop in Almere Centrum unless they are an express train. If you are coming from Schiphol Airport, look for trains going to the above cities and they will most likely stop at Almere Centrum.
You can always ask any NS Train employee for help. All of them speak English if you do not speak Dutch.