Before diving into using Web Forms, there is some basic setup that must be performed.
By default, the Web Forms feature is available to all users. This includes the Web Form builder and Propsect Workspace. If you’d like to restrict access to a certain group of users, you can do so in Feature Security. This is an optional change.
When a Web Form entry is submitted, it is evaluated within the Prospects Workspace. If the entry is promoted to a Record, there are additional required fields that need to be added to make it a valid Record.
Each User that manages the Prospects Workspace will need to set defaults of how these fields should be filled when promoting Prospects to Records. This is done in the User Profile. Users will not be able to promote Prospects to Records until their defaults are set.
To set your default required fields:

Every Prospect promoted to a Record by the User will have these field settings applied, in addition to a Tag of PromotedProspect. See Handling Web Form Responses for more details.