How did scarecrow man meet dorothy?why he wanted to go to wizard of oz?

While on her journey, Dorothy stops to gaze at a cornfield. She sees a scarecrow an focuses on his painted face,

While Dorothy was looking earnestly into the queer, painted face of the Scarecrow, she was surprised to see one of the eyes slowly wink at her. She thought she must have been mistaken at first, for none of the scarecrows in Kansas ever wink; but presently the figure nodded its head to her in a friendly way. Then she climbed down from the fence and walked up to it, while Toto ran around the pole and barked.

"Good day," said the Scarecrow, in a rather husky voice.

He wanted to go to Oz to ask for a brain.