You'll clear passport control (immigration) in AMS and customs in ARN.

AMS is usually pretty good about shuffling people with short connections through the line from the non-Schengen to the Schengen area. Depending on the time of day you arrive, that should be enough time. I've been there with very long lines (all the way down past the ABN-AMRO exchange--almost to E gates) but it still only took 15 minutes.