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A strong starting point is Introduction to Microsoft 365 (MS-900T01-A). It is a beginner-level course designed to introduce Microsoft 365 as an integrated cloud platform and covers:
- cloud fundamentals and cloud computing
- Microsoft 365 productivity, collaboration, and communication solutions
- security, compliance, privacy, and trust in Microsoft 365
- subscriptions, licenses, billing, and support
This course is aimed at learners who are new to Microsoft 365 or have only limited experience, so it fits a first step well.
If the goal is specifically learning apps like Outlook, Word, Excel, and Teams, a Microsoft Q&A answer also points to starting with Microsoft 365 fundamentals learning content for those apps.