Agar.io is an online multiplayer game. You just need to consume smaller targets to increase in size and stay away from bigger ones to not die.
When you want to go in a certain way while playing the game, you have to move the mouse in that direction. Simply hitting the 'w' key on your keyboard will allow you to fire your weapon. If you want to get smaller and faster, the only way to do so is to reduce your body mass. You can cut yourself in half as many times as you like by pressing the space bar on your keyboard. This will make you smaller while also increasing your speed.
You can alter the look of your cell in Agar.io by going to the "name selection box" and entering a different name for it, such as "Phonix." For example. In the event that the name does not have a skin connected to it, you will simply appear as a colored cell in a random pattern. There are a variety of skins available, some of which include the Earth, the Moon, Doge, and even a CIA operative. This will allow you to take on the appearance of whichever name you type in, provided that there is a skin available for it. Good luck and become the owner of the biggest circle.
How to control:
Use the mouse.