How can I make eyelids perfectly round in order to imitate the shape of eyeball?
place a sphere inside the head a subtool and sculpt around them.
But they won’t be perfectly round!
Then dont use zbrush 
I’m not sure if this would work but try drawing out a sphere, convert it to a polymesh, select/mask the entire sphere and ‘extract’ a shell from the sphere. This should give you a hollow sphere with inside and outside mesh surface. This could then be ‘cut up’ into upper and lower lids. ~S.~
And what can I do to make eyelids fit tightly to the eyeball?
Once you have the eyelid shell, select the whole shell and scale it until it fits the eyeball the way you want. If your eyelid shell is in fact an extraction from your eyeball sphere, the two objects will be concentrically alligned so simple scaling should give you the effect you’re looking for. ~S.~
but eyelids and the head won’t be a one object!