For representing characters we use 7 bits in a so-called ASCII (American Standard Code for Information Interchange). Many people say "ASCII code", but that is redundant. Ascii tables can be found at www.asciitable.com..
An excellent article on the history and expansions of ASCII and UTF can be found in the wikipedia article.
Bits can also be used to represent sound: sound consists of waves, which are measured (say) 10000 times per second: the height of the wave is then represented in the computer.
For images, the image is decomposed into "picture elements" of "pixels", a rectangular array of them is formed, the color of each pixelis then encoded and stored.