How to make ubuntu fullscreen all the time

It is not quite easy, and options are limited. Unfortunately, Linux desktops provide limited user control on the default size of newly created windows (I do not know whether the situation is any better in other OS´es). Then also, there is a difference between "maximized" and "full screen".

Suggestion 1. devilspie2 is a daemon that monitors the creation of windows, and automatically resizes or repositions them as they are created. You could configure it to full-screen any window, but then you would need to define exceptions such as File or Print dialogs, or any other window you do not want to be maximized.

Suggestion 2. Per application approach:

If you can live with "Maximized" rather than full screen, some applications remember their previous state. Examples include Firefox, Files, Text editor Gedit, Gimp, Inkscape

Other applications provide command line switches, for example Evince document viewer: evince --fullscreen, Gnome terminal: gnome-terminal --full-screen. For these applications, you could change their .desktop file to add the necessary command line arguments. A more robust approach could be to have your own wrapper scripts, which then would call the actual executable with the needed argument. The latter approach would not only work if you launch the application from the menu, but also when it is being launched from within another app, e.g. Evince from within Firefox.

This way, you should be able to set up a working environment where most applications by default open in a mximized state or at least full-screen. Not perfect, however, by all means. An approach with devilspie would be more perfect, but more difficult to setup.