I am just researching on it and I have found that if you shoot the Drake's Tail 20 times it will fall off resulting in gaining the Drake Sword. Is there an easier way to get it or is it the only way?
The easiest way to get the Drake Sword is to just shoot the tail with a bow from under the bridge, the simple reason for this being he can't attack you back. If, for whatever reason, you do not want to do this you have a few other options.
It is possible (though difficult) to aim magic even when you are not targeting your opponent. If you get good at this you could use magic (such as soul arrow) instead of a bow to cut off the tail from under the bridge.
If you have a bow or other long ranged weapon you can aggro the dragon from under the bridge. He will fly onto the bridge and stomp around a bit. During this time his tail is within melee range of a long thrusting weapon like a spear. Be careful or you will fall off the bridge.
Finally, you could take the dragon head on. Aggro him while on top of the bridge then run right at him and try to hit his tail with a weapon of choice. I do not recommend this route as surviving his fire is very difficult early on.
A specific technique that a lot of speedrunners use, while dangerous, can be used right from the get-go.
I advise that you are under 25% carry weight, so you can run like hell when you succeed!
You will need the Gold Pine Resin (Lightning powder - you find it in a chest before you take on the Taurus Demon on the bridge). After the Drake lands, go down to the bonfire and rest. Come back up, and wait on the side of the bridge, out of harm's way.
Apply the powder, and wait (I would also 2-hand your weapon, for extra damage). The drake will eventually jump off, and land on the bridge. Now is the time! Run like hell for his tail, and take a good couple of swings. 2-3 swings will be enough to cut off the tail, then run like hell for the bonfire at the end of the bridge.
I would suggest hiding off in the balcony to the left until the Drake flies off, then you can rest at the Bonfire.
Here is a video of the run, (but this guy doesn't do too good of a job)