Hi @Caio_Silva !
Welcome to the community !
Whoaaa xD You are adventurous with such old hardware.
Do you need to get inside GUI ? or only command line text mode is enough for you ?
If you are still before installation of UM, during the initial setup inside user information, you can check the "Auto login" check box.
If you need to get to the GUI, after installation, go to the menu and search for "Login Window", it will prompt for your password, next go to Users and configure auto-login 
If you don't need GUI, try more something like Ubuntu Server and fallow this link to configure auto login
I have a old IBM X40 computer - 30 years old now xD (pic here) perfectly running UM 18.04, fully up to date too, and like using it sometimes for cool stuff like web browsing
Good luck