According to the Wiki Itself
A bow will likely be the best weapon for dealing with a witch, as the splash
potions have only a short range (at most nine blocks) compared to arrows (which,
when fired straight at full charge, travel 24 blocks). If a bow is not an option,
a direct melee charge is the best option. Due to the witch's slow attack speed, if
the player runs in a straight line and manages to get a quick sword strike in,
the witch will try to heal themselves with a potion of healing, leaving the player
to attack them without fear of retaliation.
So by the sound of it, if you haven't yet got a bow to fight (which early on you probably don't) the best option is to dodge their first attack, then charge in swinging your sword. With a charge, your first strike will probably be a crit that will force the witch to heal, and then you can barrage them with sword blows until they're taken down.
This might be counter-intuitive with other mobs around, since rushing in is the last thing you'll want to do against them. It would probably be a good idea to take out any wandering mobs first before dealing with the Witch, if they're all gathered together. If they're not, then it'd be best to charge the witch and take her out very quickly, before running away from the rest to attack at leisure.