The Controls in Slither.io are relatively simple. On browsers, your snake is controlled by your mouse and moves in the direction of your cursor. Your snake can also be controlled using the left and right arrow keys on the keyboard. On mobile you simply press and hold in the direction you want to go.
You can either click and hold the right or left mouse button, hold the up arrow or W key, or hold the space button to generate a burst of speed (in mobile edition, you have to double-tap your screen). This makes your snake move significantly faster than usual at the cost of shedding 15 of your current length per second. However, the amount of length shed remains constant, meaning that the larger your snake is, the less significant this loss of length is. However, when using the speed boost, it can be harder to control your snake precisely and you'll have less time to react to the movements of other nearby snakes.
A snake using its speed boost can be easily identified by the shining/shimmering effect that is applied while speeding.