Imac G3 1999 and monitor problems

[Groovy]

I have installed the YDL 4 version on an Imac G3 1999 (blue) and it
works,but I cannot start the graphical mode. I can only use the text mode.
When I try to start the X window interface, I got an error and I anm thrown
out from the graphical mode. How can I set up the display settings from the
text mode? I am a newby and I don't know a lot of Linux commands. Please
help me!
The Groovy from Italy