Here are some tools provided by Microsoft to assist with migrating SQL Server databases.
Microsoft Assessment and Planning (MAP): This tool can scan your network to discover all of your SQL Server instances. Once the tool gathers the information (it will store that data in a local database you set up) you can report on your instances by version, edition, etc, as well as give you information on the host servers, counts of database objects per instance, among other metrics.
Data Migration Assistant (DMA): This tool can analyze a specific instance and inform you of any issues you could run into when trying to upgrade to another version of SQL Sever, as well as issues with moving the instance to Azure. This tool will report on any deprecated or removed features, breaking changes, as well as give recommendations on new features in the target edition. The tool can also migrate the databases and logins for you, using backup and restore. The tool can be used for editions 2005 and later.
Database Experimentation Assistant (DEA): This tool will capture a workload on an instance and replay that workload on a target server. The tool will compare the two workloads to discover any queries where performance degraded, or any other errors or issues found. This is useful to get a preview of what a workload would look like on the target, which could be a different edition, on different hardware, among other changes.
SQL Server Migration Assistant: This tool is to automate migrations to a different data store. As of now, there are tools to migrate from SQL Server (2012 and higher) to Access, DB2, MySQL, Oracle, and SAP ASE .
Azure Database Migration Service: This service will analyze on-premises databases (using the Data Migration Assistant) and will perform an online migration into Azure services. This tool is part of a larger suite of tools, Azure Migrate.