Hello Nida,
How wonderful for you to have found the
Afternoon Tea at Disney's Grand Floridian Resort & Spa. The Garden View Tea Room is almost complete with a recent refurbishment and I can't wait to visit again. The easiest way to arrive for tea is to take the resort boat launch from the front of Magic Kingdom Park. After exiting Main Street, U.S.A, you'll find the resort boat launches off to the slight right side of the exit. Disney's Grand Floridian is the first stop, making this transport a very easy one! You may wait for about 10 or 15 minutes if you missed the last boat, but the trip across Seven Seas Lagoon is only 10 minutes.
I would suggest heading towards the Magic Kingdom exit 1 hour before your Afternoon Tea reservation. This will likely provide you 30 minutes or so to explore Disney's flagship resort and take in the Grand Floridian Society Orchestra, playing daily in the lobby beginning around 3:00 p.m.
It looks like you've already discovered Advance Dining Reservations are highly recommended. You can book online or by calling 407-WDW-DINE.
Cherish this experience, Nida. It's a delightful one.
Best wishes,
Sherri