Valentine is telling Viola that if Orsino keeps treating you so well, then Viola will lead a good life.
Does Orsinos' feelings change suddenly?
Orsino has known you for only 3 days, but he already thinks high of you. 
No
Orsino asks for Viola and wants her to go to Olivia's house.
If she is in depression, then she will never let me in.
Go to Olivia's house and if they do not let you in,  plant yourself outside her door.
Orsino tells Viola what to do if Olivia does not let her in.
Lets say that they do let me in.  What shall I say?
Be loud and unpleasant. Just get the job done.
Orsino tells Viola to tell Olivia how much he loves her.
I don't think so, My Lord.
Tell Olivia how deep my love is for her.  Tell her examples of my loyalty.  She will pay more attention to a young man like you than to an older, more serious man.
Orsino talks about Viola's feminine features, but he still doesn't realize that Viola's actually a girl.
I mean, just look at you, you!  Your voice is so high and you almost look like a girl.   But it's alright, I know your a boy.  Now go woo Olivia for me.
Uhhhh.
The readers find out that Viola actually loves Orsino!
But... now I have to be matchmaking someone for the person I love!