I just bought GMC Safari 91 and before that I never drove car with automatic gear.
So maby somebody can explain me what is the difference between D, D in circle, 2 and 1?

uhh.. you are serious right?
Anyway.. the D in a circle is normal driving mode, it allows the overdrive lock up to operate giving you good mileage and less engine speed. The D is for when you don't want the transmission to keep shifting back and forth say while going up a long hill.
The other gears are sort of like the same as using a manual transmission to keep the van in the same gear. The only times I use the lower gears is when I am towing a trailer.
I would not suggest using the gears like you would a standard transmission car for everyday normal driving.