i need help

I can not find the room of a friend.

You'll need to have them send you an invite via email or some other communication mechanism.

How do I send a friend request?

Roll20 doesn't have the concept of a friend, merely games you are a member of.  it does show you the links to the profiles of the last 10 people you've played with in your own profile, but if you haven't connected to them yet, you won't see anyone. Once your friend has made a game, they can get an invite link to share with you via some other mechanism (email, social media, carrier pigeon...).  They can also invite you directly using your email address.