Playing games, harvesting berries. Basically doing activities.
You will need to trade it from Pokemon White. Or you can get it at Spooky Manor in the Dream World (Over 10,000 dream points).
Lopunny cannot be found in Dream world, but Buneary can-In the pleasant Forest after earning at least 7500 dream points
the dream world is where you can get pokemon from different generations.
You need 10,000 points in Dream World. When you reach it, Ralts will be available in Spooky Manor. Go for it! You can find and befriend it. Good Luck!
You may need some more dream points, for example tropius needs 1,700 dream points to find
You can plainly get any Pokemon in the Dream World if your patient and to continue searching.
I think that in the Dream World, you get points when you do certain actions (I forgot, go to bulbapedia and type in Dream World), but I think when you reach a certain number of points, you unlock new area, meaning new Pokemon, too. I haven't done it myself, but I read it somewhere so I'm not sure.
In the Dream World, you will automatically have 2 Garden Bed rows however the 3rd row requires 900 points and the 4th row requires 2,100 points. For a 5th Garden Bed row, you will need to get at least 3,500 Dream Points. The 6th bed will require 10,000 points, the 7th row requires 20,000 points, 8th row will require 30,000 points, the 9th row will require 50,000 and the 10th bed will require 100,000 points.
that's on Pokemon black
20
Usually 5 to 10 minutes
On the website, on the login page, there's a Profile button. On your profile, you can see your Dreamworld data, including the amount of dream points you have.