Amsterdam and Diemen are separated by the A10 ring-road, the exit Amsterdam-Watergraafsmeer can be used to access most parts of Diemen. Diemen can be reached from The Hague, Rotterdam and Leiden using the A4 followed by the A10 ring-road. Utrecht, Den Bosch and Eindhoven can use the A2, followed by the A10 ring-road. Apeldoorn and everything north there of is best off using the A1, exiting at the exit Diemen or continuing on the A1 followed by a short section A10 and exiting at Amsterdam-Watergraafsmeer, depending on where you want to be in Diemen.

Tram line 19 goes from Sloterdijk via Leidseplein, Weesperplein to Diemen. It is a 30 minute ride from Leidseplein. If you are coming from the Central Station the best way is to take the metro  51  53  54  to Weesperplein and from there on take the tram to Diemen, as there is no longer a direct tram line from Central station to Diemen.

Diemen also has two train stations: Diemen and Diemen Zuid. You can take the  SPR  from Amersfoort to 52.345164.967561 Station Diemen.

As Amsterdam is the bike capital of the world, the best way to explore Amsterdam and its suburbs is by bike. It is approximately 7 km to Diemen from Dam square. That might be a long ride but you also see parts of the Plantage and Oost neighbourhoods.