In this short how to guide, you will see a quick run through of the steps to take to create a group in Discourse.
There are 2 ways to start the group creation process, you can either go to /g aka your-site.com/g or you select the hamburger menu from the top right part of your screen and click the Groups option:
From there you click the 
New Group button also at the top right part of your screen. Clicking this will open a new page with which you can use to create and configure your groups to your preference.
For more details on the different details on how to configure and use the groups feature to your preference, see these other how to topics here: Latest groups topics in howto - Discourse Meta. As a beginner, the two that would be of the most use to you would be:
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How to set / configure private / closed group access, membership and visibility, this helps you understand how to create private groups and membership groups.
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How to set and use group mentions and messages, this helps you understand how to interact and engage with a group using mentions and group messaging.
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How to create private categories using category security settings, this is a more advanced setup that shows you how to use the category feature and groups in Discourse to create private sub-forums within your larger forums. These are like private channels in Slack and Discord.