When referring to the homepage, a trailing slash after the hostname is optional since it leads to the same content ("https://example.com/" is the same as "https://example.com"). For the path and filename, a trailing slash would be seen as a different URL (signaling either a file or a directory), for example, "https://example.com/fish" is not the same as "https://example.com/fish/".
Another excellent guide is Google’s “Search Engine Optimization Starter Guide.” This is a free PDF download that covers basic tips that Google provides to its own employees on how to get listed. You’ll find it here. Also well worth checking out is Moz’s “Beginner’s Guide To SEO,” which you’ll find here, and the SEO Success Pyramid from Small Business Search Marketing.