Set the horizontal CG by placing your model so that the world location 0,0 is at apx. 25% of the wing chord from the leading edge.  Vertical CG should ideally be through the centre of mass, usually through the prop spinner but this depends upon type.


Gnome

"Typically" a modeller will set the CoG of the model to be the same as the CofD (Center of Data).  IT DOES NOT HAVE TO BE THIS WAY!

I've noted that some of the default FS models have their center of data (the infamous 0,0,0 point from which all measurements in the air and cfg files are referenced) at the nose of the airplane.  (As an aside: In reading some of the US Type Certificates, and other technical information, many times the reference point is not the CofG, but rather some defined point in the structure)

If you have a compiled model and you're not satisfied with the CofG, then you can move this in the aircraft.cfg file, in the appropriate line (I don't have any references right now, it would have to wait until I get home).

Remember - aircraft.cfg measurements are from the data center, not the CofG.

I don't understand what you mean by "center of the viewport"?

Also, what do you mean by "starting the textures" ?  Is this a separate issue?  Texture assignments are not tied to CoG/CoD issues.