So ...

Upon activation, go to Tablepress => Add New Table.

Define the name of your new table, add a description and select the number of rows and columns according to your needs.

Needless to say, you can always add/remove columns and rows to/from your table when entering data or by editing the table later.

When you are happy with your choices, click the "Add Table" button:

You'll be taken to the table editor where you need to add the data to your newly created table:

Also, you can add or remove columns or rows and of course, all rows and columns can be dragged or re-arranged. An you'll see a bunch of additional formatting and displaying options that will allow you to customize your table according to your needs. Finally, a shortcode will be automatically created, allowing you to easily insert your table in your content ...

And don't forget to click the "Save Changes" button!

OK. Now let's see how can you insert the newly created table in a post or a page ...