Bus 312 and 316 run from Amsterdam Central Station to Volendam and back, approx. 35 minutes. A one-way ticket costs 4.39 EUR with OV-chipkaart.
Bus 110 runs from Station Purmerend to Volendam and back, approx. 35 minutes. A one-way ticket costs 3.45 EUR with OV-chipkaart.
To get to Volendam from Station Hoorn, take bus 314 and change to bus 316 in Edam, approx. 45 minutes. A one-way ticket costs 4.40 EUR with OV-chipkaart.
Book with a travel agency
Various travel agencies offer day tours from Amsterdam to Volendam, often combining this with a visit to Marken and Zaanse Schans. These can be relatively expensive, but at least you will be sure to see all the major sites within a single day.