Question

the city that was decided as the starting point for numbering meridians

Answer 1

A meridian is required to number the other meridians.  

So, in 1884, it was decided to keep Greenwich, a place near London as the starting point for numbering meridians.  

The meridian passing through Greenwich was named the Prime Meridian and was marked at 0 degrees.  The longitude of a place is measured east or west of the Prime Meridian.