You can buy a ticket in the bus. In the Amsterdam regio you can buy a one-way bus ticket for € 6.00 (from 1-1-2019: € 6,50). Children aged up to 11 accompanied by an adult can travel for € 1.00 per journey.
Are you a frequent traveller on Connexxion buses? In the Amsterdam region, the Amstelland Vrij subscription may be an attractive option. Monthly or annual subscriptions are available. Another option is the Randstad Noord Zone Abonnement which is valid in several area’s and more operators.