How can I improve my skills in shop drawing? How can I learn more about codes that used in shop drawing?

Practice drafting your own project or any other project which is already been done,

use the shopdarwing which was already done as your reference to review accordance and compare the detail required to build or construct.

shop drawing is just giving you more detail and corrections to build or construct.

1-Read and understand the design and original sho drawing.

2-draw and compare with the original shopdrawing

you should know the method statement and the specifications of the item you want to make its shop drawings also you should have a site and technical background 

The skills in shop drawing can be improved by diversity in projects and frequent practice of specialization

Graphics are done using autocad and autorebar

If you are using AutoCAD for drafting, the fastest way to use it by practicing keyboard input for tool selection.That way you can use your both hands simultaneously and reduce the time. The key board input assigned for individual keys are available online. Rest is muscle memory, more you practice more it gets better.