startx with no screen found!

[Mohammad Badie Zadegan]

Hi everybody,
I was used OpenBSD 5.7 two years ago with vesa driver without any error but now 
when I come back to OpenBSD I install the latest release and saw that some body 
work fine with nv driver but I get no screen found when type startx!
How can I resolve my screen issue?
I prefer installing nVidia on my OpenBSD but if I force to vesa, How can I 
select vesa screen mode?
Thanks for your helps.