You fell into the underground when you were little, and was adopted by Toriel, The caretaker of the RUINS, she took good care of you, giving you a home and even a butterscotch cinnamon pie every now and then

One day, you were sitting under the large leaf-less tree, reading a book in front of Toriels house when a small vine wrapped around your leg, and then a small flower sprouted... With a face..?

Flowey: "Howdy Stranger! I'm Flowey, Flowey the Flower!'