Valla is one of the most easier hero.
Basic attack build:
Your only job to keep hatred and shoot closest heroes. The easiestway to do it, use attack move [A] often. If there are creeps, its for keeping hatred. If there are no creeps, u will automatically attack closest heroes.
NEVER dive! Always keep distance!
Hungering Arrow build:
The best if there are only heroes or may better if only one. Shoot Multishot, Hungering arrow, refresh with Vault, shoot, refresh, shoot. This build is working with only almost all of your mana. So you have to go home often!
Multishot build:
This build is effective against a whole team, and with more heroes with executioner talent.