Hi ,
This is quite a broad question, and needs a lot specificity to clearly guide: Here is a link to get started on how to prepare to got to the cloud. -> https://learn.microsoft.com/en-us/entra/architecture/road-to-the-cloud-migrate.
Here is a best practice guide based on Zero Trust https://learn.microsoft.com/en-us/microsoft-365/security/microsoft-365-zero-trust?view=o365-worldwide