If you just bought Varus or trying him out on free to play I will explain to you what his spells do when you just started a game.

Passive. When Varus will kill a minion or monster Varus will get bonus attack speed. When killing or assist a enemy champion he gains even more bonus attack speed for a longer duration

Q. Your Q You should max first your Q does massive damage, when you tap Q Varus will start to ready his arrow, you'll have a few seconds to launch the arrow by left clicking if the time runs out he won't fire the arrow and will return the mana that you used and put your Q on cool down. This is supremely good at poking enemies at a long distance. The arrow will pierce though other enemies and reduce damage when the arrow is going though

W. Your W adds plus magic damage when you auto-attack it'll charge up to three times more charge your other spells when used will add more damage

E. Your E is when Varus shoots arrows in a certain area which will damage enemies in that area and when in the area will slow enemies and reduced healing.

R. Your R is a skill shot that when hit at a champion will snare that person and people around the hit champion with magic damage

I chose those runes because having a lot of armour penetration when a tank builds armour it'll go though the armour plus all the attack damage that Varus will have with his Q adds massive damage with the last whisperer late game with runes and the critical chance will add even MORE damage to the enemy

I chose these items because blade of the ruin king a bloodthirster will add some life steal and varus doesn't have and life steal with his spells and the critical chance with armour penetration is very suitable for Varus to have

During team plays when everyone goes in use your ult with E to slow the enemy plus snare which when enemy falling back will slow them but the snare won't let them get away. This is helpful stealing baron Ult then Q so you'll be able to steal when the baron is low E so the enemies will have trouble getting away then your team and you will finish them off