edit ms CRM 365 interface using vs 2015(CRM SDK)

Hi, I`m new in developing CRM 365

I learned about plugins and I couldn`t find anything about changing the user interface of CRM 365 with plugins with visual studio.

for example, add a button to the top menu in invoice page?

I was thinking to make a plugin to clone invoices but I can't find anything about how I can add a button to the invoice page and execute my plugin with that.  

I just wat register my plugin to CRM 365 and my custom button appears automatically. 

thanks for helping me guys.

(I work on CRM 365 on-premise)