Can I create a new character without joining a table?

Yes, you can!

Select the system you wish to create your character for.

To do this, go to Shop, click on System and select a system from the list.

In the window that opens, click on the blue +Create New Character button.

Assign a name to your character (you can change it later) and click on the white Create my Character button.

Then create your character according to the system rules and/or your GM's guidelines.