Hello i'm Aklautos and this is a simple guide for Valla. Valla is my main champion a long time now and i just hit level 20 with her some days ago with over 250 games in my quiver. I'm currently a gold ranked player in Hero League so my suggestions only include my experience pretty much on this level.

Although Valla is a standar ranged assasin with pretty much easy skill set, choosing wisely when to use your skills makes a lot of difference.

Pros :
1) Deals easily a lot of damage (but not burst)
2) Can chase down enemies and get kills
2) Good mobility great for poking and disengaging
3) Can be used as a crowd control (cc) player

Cons :
1) Poor health pool makes her easy target if focused or cc'ed
2) No burst damage as mentioned before

Level 1 : Hot Pursuit is always my pick as it gives you solid movement speed bonus plus mana regeneration which is really needed for Valla. Both other options don't fullfill your role as a "killer assasin"

Level 4 : Poke heroes as much as you can when you get this talent untill you finish the quest. Passive is also nice. Great bonus for your damage output

Level 7 : I know most people don't choose this talent but many times it's the difference between a kill and an escape. Great for burst also when 1v1 and nothing around you

Level 10 : The ult i choose in 80% of the games. Great for cc, great for combo's, for escapes and for 1v1. What else do you need ? Choose if the enemy team has too many squishes or if you wonna hafe fun in some games :P

Level 13 : This is purely for some sustain during teamfights.

Level 16 : Second great choice for some cc. Can be used for chasing and for escaping

Level 20 : Really nice talent gives you the ability of non-stop auto attacks.

Valla is a really nice assasin who can bring the terror to the enemy team if used correctly.
The most important thing to remember and pay attention while playing her is the positioning. If you stay too close to the enemy team you'll die fast and won't be able to contribute for your team. If you stay way back afraid of getting nuked you won't do enough damage. Perfect spot is right behind your tank/mellee assasins and in front of your back line mages.
As mentioned before the skill set is easy but !!!
1) Don't use to engage as you won't be able to escape if needed. Use only if chasing for a kill or you are sure you have another escape plan
2) Don't use or at the start of the fight. Let them think they you dont have any damage in you untill you suddenly mini burst them.
3) Your team is really important for you. Don't pretend to be a hero and jump in front of them believing there is gonna be a rescue. If you see your team retreating even if it's wrong don't go solo,chances are you will end up dead

Valla is a really fun champion to play. She can be selected either for ranked or for fun plays in quick matches. This is my first guide so any suggestions or corrections are appreciated ! I hope this guide will give you a competitive build to play or at least some food for thought :P
Good luck, have fun and remember "Let none survive!"

Here is a link to my hotslogs profile : https://www.hotslogs.com/Player/Profile?PlayerID=6059582