Going that distance gets to be a professional project very quickly since there are many factors involved.
This antenna is one of the largest you can get. It will not be the antenna that costs the money though.
You now have the problem of what do you attach to the antenna. You can either buy expensive microwave cable to bring the signal inside to a cheap router/radio or you can buy a expensive waterproof router/radio to mount outside on the antenna.
For performance reasons you are better off mounting the electronics on the back of the antenna. Does not help a lot if you buy a huge antenna and then lose all the signal in the cabling. If you are handy people do mount consumer routers outside in home made enclosures but it does not work well if you live in a area where the sun will cook the electronics in the summer.
You will want electronics that has the greatest transmit power and best sensitivity to receive. Be aware there are legal limits on how much power you can transmit which varies from country to country and type of antenna.
You also want to be up as high as you possible can. When you go the distance you are talking about the ground tends to absorb signal.
hello, i would like set up a wireless connection with a friend of mine who lives about 20km away. I would like to know which antenna's I should get and how to set them up. There is a clear line of sight from my house to his. The connection will be for file transfer and gaming, so it needs to be quite fast. any advice would be appreciated