I was skimming through the Writer's Digest website and I found two articles on minor characters. You can read the articles here and here. I have a bad tendency to vomit minor characters onto the page in my stories and then expect you to remember who everyone is. I also used to have a habit of naming characters with similar sounding names so it was easy to confuse who I was talking about.
Minor characters are important. Unless your character is all alone and doesn't interact with anyone, you're going to have minor characters. Whether they're just walk ons and part of the scenery or someone your readers will need to remember depends on what exactly you want them to do.
Walk ons are just that. They're part of the scenery. A guard, a cook, a maid, a valet, a priest. All of these characters can share scenes with your main character, but they don't play a major role. A line of dialogue or just having them there as part of the scenery to give us an idea what the stage your main character is playing on looks like, these characters are so minor that they'll pass from your reader's mind fairly quickly.
Then there are the minor characters that your main character deals with on a more personal level. A parent, a lover, a husband, a wife, a child, a best friend, a traveling companion. Your main character's going to have more contact with them than a brief passing in the hall or casually noticing them in the room. They're going to be named with at least a minimal amount of description and a few lines of dialogue. These characters you can make stereotypical, or you can make them stand out and break the stereotypes to make them three dimensional characters in their own right.
Your character is going to interact with a plethora of people in your story. Make sure the people who are important are memorable and those that are just part of the setting are unremarkable. Don't pick names that have similar sounds or are the same number of syllables. Give those characters that your main character interacts with regularly some personality quirks so they seem like real people instead of cardboard cutouts.