Some can ripen at home.
Others, well... buy ripe or you are committed to eating unripened fruits, i.e., they will not get any riper.
Fruits that will ripen after they are picked are known as climacteric fruits. They include:
avocadosFruits that will NOT ripen after they are picked are known as non-climacteric fruits. They include:
applesTo nudge climacteric fruits to ripen, place them in a paper bag with a ripe fruit. This will hasten the process.