Turgensec posted a great user guide to using Shodan. You can find it at https://community.turgensec.com/shodan-pentesting-guide/. Topics include the following.
1 What is Shodan?
2 Shodan interfaces
3 Shodan Search Query Syntax
4 Shodan dorks & use cases
5 Tools using Shodan
6 Articles of advanced uses
7 Shodan alternatives
For those that are not familiar with Shodan, think of it like this. Imagine 20 something servers constantly scanning the Internet for anything and collecting information about those things from service banners. Then imagine a Google like search bar created to search for such things. Shodan can be used to find anything from a specific Vulnerability landing page (ideal for those looking for vulnerable systems accessible online) to a camera login page (ideal for those looking to log into webcams that do not have passwords. It’s an awesome reconnaissance tool you should check out if you are a pen tester researching a target OR interested in what attackers / pen testers could find out about you and your organization. Check out this guide. It’s good.