There are several ways to add new items to lists.
To make a quick list, tap the green button next to the list name. It will add a new section named after your list that you can use for your items.
To make a better-organized list, tap the button in the lower right corner, and it will give you a screen that allows you to add an item in a new section. Once you have one or more sections, you can use the green button next to each section name to add new items to that section.
To add many items at once, you can use the Group Add button, which will be covered in a separate tip.