If you want to get access to MailChimp API, you will need to create an API key from their site. To create your first API Key is so simple, in this tutorial you will learn how to create the API key for yourself.
Once you have login into MailChimp website, under your profile account menu which is located on the far top right, there is a drop down menu named Account.
It will then loads your account overview page, under the extras menu, there is a menu called API Keys.
Under the API Keys list, there will be a button called Create a key. Click this button will automatically generate a new API Key for you.