How can computers represent texts as numbers?

Assuming you're talking about storing text as binary digits in a computer's memory or hard-drive etc... Each character the computer is capable of producing - is given a numeric value. The ASCII code (as it's known) is standard worldwide - and assigns the number 65 to the letter a, 66 = b, 67 = c etc. These numbers can then be converted to binary by the computer for storage. After conversion to binary - the letters a,b & c are represented by 1000001, 1000010 & 1000011 respectively.