Netherlands the land of the bicycles you could say. There are more bicycles in the Netherlands then there are people living in the Netherlands. The Dutch people learn to cycle from very young age and are used to do as many things as possible with the bicycle.
Renting a bicycle
For renting a bicycle there are several options dependent on your needs. If you need a bicycle to go from a central train-station to a location and later you come back at the train-station you can rent an ov-fiets. It can be good for easy bike rides and short distances. Expect a sturdy heavy bicycle which just works. The bicycles have good locks also.
Another popular possibility is to rent a bicycle for longer term, for this you can use Swapfiets. In the bigger city's in Netherlands this service is available. They rent a range of bicycles from simple until e-bikes. You will pay an amount monthly. In this amount is included service of the bicycle, insurance and locks for the bicycle.
This option of renting I would recommend if your not sure yet if bicycling is something you would do a lot. Since many bicycles are stolen each day it's a comfortable feeling insurance and locks are already included. If the bicycle will break down, the swap the bicycle for a working one. This is the reason for the name Swapfiets.
Buying a bicycle
For buying a bicycle you can go to a bicycle store. Usually they will have many different types of bicycles from which you can choose from. A lot of times they will have used ones as well. The advantage of buying from bicycle store is the additional service they can perform. They can include some maintenance in the price as well.
For a use bicycle there is another option to look at the biggest secondhand online marketplace called marktplaats. Its a bit more risky because sometimes they advertise stolen bicycles as well. But you could ask the seller if they can show the receipt from the time they have bought the bicycle to tackle this problem. Usually it should be no problem to test the bicycle before buying. It's good if you are able to spot any problems which the bicycle might have otherwise it could be wiser to buy a bicycle from bicycle store.
Lock your bicycle
Many bicycles are stole each day in the Netherlands. Make sure you invest in some good locks and attach your bike-frame to something preferably.