If you want to input your leads manually, you first need to create an empty list. Go to Manage => Lists => Add a new list, click on "Empty list", type in a name for your list and hit "Next".
Once you have your list, you'll need to define some fields to it to be able to add leads. But that is all explained in the next section of this guide: Tweaking Your List.
Where does your data come from?
If you have data from other online systems, check out our guide on integrations for inspiration.
Create an empty list and configure it to upload leads manually.