Where to start : i want to create unity extention which streamline the work

Hello all
Im not coming from unity echo system so please forgive me if the questions are not belong to here.
im checking the possibility of extending the Unity editor to streamline the work .
it means i need access to game objects , adding new menu items to the editor , adding some new editor windows .
in short adding new layer on top of unity .
questions :
1.where to start ?
2.to i need to buy Unity license to be able to create Unity Extension ?
Thanks

 

You don't need a paid Unity license. You can extend the editor with the free version.

See:

• Extending the Editor (manual)
• Building a Custom Inspector (video tutorial)
• Catlike Coding's Editor Tutorials (external; a bit old but still useful)

 

Thanks allot !
And to publish to the Assent store do i need license ?

 

No. More info here.