I want to give you a serious answer.
Virus for android are generated with msfvenom in Metasploit Framework, learn Linux OS and Metasploit.
For more information read this: http://www.kalitutorials.net/2016/01/metasploit-for-future-hackers-msfvenom.html?m=1
Do you wanna create a virus from 0? Learn a programming languages like C++
Test the exploits with YOUR device, trust me... it's not fun harm others people.
Hacking != Virus in the phones of your friend.
~Sorry for my English ^_^