18 is well past old enough for you to know your own sexual identity, it's probably not a phase and people truly can be one gender on the outside and another on the inside. It's not a disease, it's just something that happens. If you want to be the best father in the world to this person who is probably having a very stressful life and needs your love and support more than anything, you need to accept this and embrace it. I promise the discomfort and awkwardness is only temporary and the sooner you embrace it the sooner you'll all be happy and comfortable again.

Quickly as possible, accept that your plans for your son are quite likely over. Make new plans for your daughter. You will be sad, there will be a mourning period and this is natural. Almost any big change will spark a feeling of loss in your life, just be happy that she was comfortable enough to talk to you about it. That means she loves you.

You are a man, you probably can't help her as much with looking more feminine, so be supportive of letting your mom take daughter out daughter shopping for whatever she wants, go along and be supportive and happy. Call her a she, and call her by her new name. Don't say anything negative even if "he" has a very hard time being a "she" at first, and it seems unnatural. These things sometimes take time. You wouldn't criticize your other daughter about a flaw in her appearance that she can't control, so don't do it with your new daughter.

And I highly recommend taking her to see a therapist. Not to change her, but to talk to her about adapting to society, meeting romantic interests, and accepting herself. These are hard issues and make it clear that is the reason why you want her to see a professional, so that she's happy in her new life.