Best if you have at least a simple sword (Stone would be the best for early minecraft days) and the witch is a bit far ( 20 - 30 blocks) than rush towards it and start hitting. Don't Stop!
Suggestion: Get her in a space where at least three sides are 1 up and attack from the opposite side where it is 1 up. Jump and when you drop hit it (Causes Critical Attack!) You may die so don't take any valuables. The witch explodes after getting killed, run away from it!