Learning Dutch depends on factors like your nationality, and whether you yourself feel it’s necessary. Since most Dutch people switch to English as soon as you start talking to them, you won’t generally be encouraged to learn or speak Dutch, and it might even make it tricky to learn Dutch when you do actually want to.
Even though it’s not always compulsory to learn Dutch when you live in the Netherlands for an extended period, it’s of course useful and fun. For instance, do you get why everyone’s always talking about the weather? Or why it’s a good idea to make an appointment with someone before visiting them?
Learning Dutch makes it easier to understand the Dutch culture, habits and quirks, and it’ll also help you connect with other people faster. Because even though you’re pretty good at English yourself and nearly everyone speaks English to you, Dutch people do prefer to speak Dutch to each other. And if you do learn a bit of Dutch, you’ll get loads of compliments, as the Dutch always really appreciate you speaking Dutch to them.
A lot of expats find Dutch a pretty hard language to learn, but it does depend on what country you’re from and which other languages you speak. For instance, if you already speak English, German, Swedish or Afrikaans, you’ll learn fast. But if you only speak Arabic or Chinese, you’ll probably find it a lot harder, as the differences between these languages are so big.
The best way to learn to speak, read and write Dutch well is by taking a language course. There are various options open to you.
Many expats moving to the Netherlands already have a job with a Dutch employer or an international employer with offices in the Netherlands before moving here. These employees often offer Dutch language courses run by external providers. These courses can be either one-on-one or together with colleagues ("in company").
There are a great many organisations that provide courses in Dutch as a second language at different levels. Sign up for a language course near where you live, or Google "Nederlands leren" (learning Dutch) or ‘NT2’ (the Dutch state exam in Dutch as a second language).
Public libraries in most towns and cities offer language buddy programmes that you can sign up for. These buddies are volunteers you can practice with on a regular basis to learn to speak, read and write Dutch.
You can also learn Dutch online. A quick Google search will direct you to some great resources for learning Dutch.