ConvertKit is a solid email marketing and automation platform. The ConvertKit platform is very easy to integrate with WordPress.
There are a number of plugins to be able to integrate with ConvertKit, also you can integrate with Contact Form 7, Gravity Forms, and WooCommerce. ConvertKit does also integrate with Memberpress, aMember, Easy Digital Downloads, and Paid Memberships Pro.
After you have created your account in ConvertKit you will be able to find your API credentials.
The main ConvertKit plugin settings can be found from;
Settings > ConvertKit
If you also have the WooCommerce ConvertKit plugin you will be able to find the settings from;
WooCommerce > Integration > ConvertKit
To get started with using ConvertKit you can follow this guide. Even with using a free plan in ConvertKit, you can get loads of marketing gains out for your site. ConvertKit will help in being able to create landing pages for your site, create site forms, create a broadcast to send to your subscribers, as well as manage your subscribers. In ConvertKit it is also possible to automate your content. ConvertKit can be used to create very complex content flows and automation.