How do I sign in or log in with tv.youtube.com/start?
Youtube.com/activate provides more than a hundred channels on it. But for that, you require an account and then sign in to the account. So following are the steps to sign in with YouTube TV;

Visit the link tv.youtube.com using the browser.
Then from the home page, select the Sign in or Login option.
Now enter your email ID and password and click on the login option.
This will take you to your YouTube TV account.