Learn Dutch: Jullie fietsen naar de stad.

Because the subject is jullie (you, plural). In Dutch, when the subject is plural (we, jullie, or zij), you use the entire dictionary form of the verb. This is called the infinitive, and for regular verbs, it almost always ends in -en.

Doesn't fietsen also mean bicycles? How do I know it is a verb here?

Yes, fietsen means bicycles, but it is also the plural verb meaning to cycle! You can tell it is a verb in this sentence because it comes directly after the subject jullie and describes the action the people are doing.

Is jullie only used for the plural you?

Exactly. Unlike English, which uses you for both one person and multiple people, Dutch specifically uses jullie when talking to two or more people. If you were talking to just one person, you would say Jij fietst.

Why do we use naar for to in this sentence?

You use naar when you are describing physical movement or travel towards a specific destination, like going to a city, a building, or a country.