Simple as I can make it:
* Planets condensed from a cloud of smaller objects
in orbit around a common center.
Envision two such objects in orbits , one slightly farther
from the center than the other.
The inner one must moving slightly faster right?
Now let gravity draw them together.
They form a new object with an orbit between the two previous orbits,
and the difference in their velocities is preserved as rotation.
(It's called 'conservation of angular momentum').
Do this often enough and you get a rotating planet.
* 'Curved" when speaking of space/time is not the same 'curved'
that you're used to in speaking of solid objects.
Gravity and the motion of the observer "curve' space in this sense.
If you're in a gravity field, you're in 'curved' space'.
If you exist and have mass, you create your own gravity field don't you?
* The center of the universe is anywhere and everywhere.
(You may consider it to be in your navel if you so choose.)
Anywhere you take an observation space/time recedes equally
in all directions from that point as a center