To answer this question first we have to understand what is light.
Light is a set of electromagnetic waves. There are much more such waves in the nature. In fact, We are surrounded by waves all the time. Every single "thing" (including human body) emits radiation, which also is an electromagnetic wave.

The light consists of waves of a particular length - only those waves are visible to human eye. All other waves are invisible to us - e.g. we can't see waves in the microwaves (those in the kitchen) and we can't see radio waves.

Light doesn't consist of only one single wave of a particular length. There are many different waves that together form something that we called "light". We can see those different waves as different colours. This is where we get to our question: why is the sky blue?

White light (or just light) consists of all colours of the rainbow. Among those, red colour has the longest wave and the blue light has the shortest wave (in fact violet light has the shortest wave, but it is less visible for us). The light, coming from the sun, gets spread around in the atmosphere (when coming from the outer space into the atmosphere there is a huge change in the density - atmosphere is relatively dense, while the space is almost vacuum). The blue light has the shortest wave and gets spread around the air much more than all other colours. This is why the sky is blue.

Why is the ocean blue?