Tableau already has example Datasets available for everyone to utilise in order to practise creating sheets and dashboards. For instance, have a look at the Sample - Superstore dataset.
It also includes geographical data, allowing you to practise creating maps and geographic aggregations. Tableau's Sample Workbooks have several examples based on the datasets I've described.
In terms of SQL practise, you haven't indicated the flavour of SQL you want to use, but you may start with Oracle, which is well-known and widely used. You may create an account with Oracle SQL Live, which will get you access to a number of Oracle-provided online example databases where you can practise SQL queries.
One benefit of utilising Oracle Live SQL is that you won't have to worry about setting up the database because it will be ready to use right away. Does that make sense?