The best way to get user ID and User adress e-mail

Hi, 
I have just install Amplitude through Google Tag Manager using the JS Browser SDK. 

I am not a developper (but a marketing guy)and I try to figure out a simple way to collect the user e-mail adress into Amplitude. 

Our application is only for connected users. So everyone has to sign in. 

I tried to look at the documenttation and I saw that :
amplitude.setUserId('user@amplitude.com'); 


1) Is it that code I need to use ? What is the e-mail variable I need to use ? 
2) If yes, can i add it to my existing code in Google tag manager ? 
3) Is it possible to use the e-mail as a UserID and should I separate it ? UserID with a number and create a new property for e-mail ? 

Thank you for your answer