Q: how i avoid an blank/grey game screen ?
What do you mean exactly? If you mean that you want it to have different colors on the default tremulous then I can't help you, but if it is doing something wrong then delete the tremulous file in "Programs (x86)" folder in the hard drive, then delete the Tremulous folder in %appdata%, and finally re-install Tremulous (I suggest that you don't install anything else)
Q: how i avoid fullscreen ?
Escape, go to settings, go to the Systems
tab, and look down the list until you find "fullscreen", then you click "apply changes".
It should reload the trem server in small box.
Q: how i change the game resolution to at least i.e 1280x1024
Same thing, it's all in the settings option in the menu under Systems