How can I empty the trash using terminal?

You can use this command :

 rm -rf ~/.local/share/Trash/*

The rm command removes (delete) files or directories.

-f, --force     Ignore nonexistant files, and never prompt before removing.
-r, -R, --recursive     Remove directories and their contents recursively.

The trash folder is found at: $HOME/.local/share/Trash

Be careful how to use rm command because any misuse may cause deleting sensitive system folders and files