If you are using a Metabase version earlier than 0.33, you will need to either write a SQL query that contains joins, or have your Metabase administrator set up foreign keys (they can read more about that here).
If you are using Metabase 0.33 or above, you can perform joins using the Notebook editor. Note that you will need to choose a field to join on, or link, the two tables together. For instance, if you want to combine a Customer table with an Order table, you might select the ID field in the Customer table and link it to the customer_id field in the order table.
