In order to get leads into Lead Assign you are going to need leads. Sometimes you already have a company, and a busy website with several calls to action for potential customers visiting your site. In other cases you are starting a new idea or company. In either case you may be running a website where potential customers will fill out a (hopefully very short) form. Upon the potential customer filling out the form, they click submit. Your form software turns this form into an email (although there are more sophisticated ways of posting information with an API or using Zapier).
This email should be sent to your email endpoint, which is simply an email address for your Lead Assign account.
Gravity Forms, Contact Form 7, Typeform, jot form, etc can all be used to add call to action forms on your website. Once you have a form setup, read the other sections about how to best format the email so that Lead Assign can understand it.