I wanted to play Varus, so i bought it like 3 weeks ago, I've checked many tutorials on how to build him but none was good enough for me.
When i wanted to sell char i've discovered quite good sequence of items for teamfights.

You have to farm all game. Search for more gold even while teamfights.
Starting from lvl (about) 13 you simply sit on your line, ocasionally enter go to jungle and harvest some frag on midline.
your statement should be like: "ok i can help but i have to go back farm".

I have 18 lvl atm so i don't really know wich runs are good :|

Varus lacks speed, so fastest boots required (earlier - better)
Executioner - reduces healing, really helps kill enemy
blade of ruined king - this item was made for varus
Ruunan's bow - atacks 3 targets at once and gives big speed boost (you have to have it)
Infinity edge - more and bigger critic. Good at later game stages

Farming is base or Varus so you should keep line dominated as long as it is possible. You can start at lvl 3:
hit 3x base atack then hail of arrows. Enemy should lose about 30-40% hp, that will force him to give you adventage on line.

Good farming starts when you'll buy avarice blade, it gives good cash boost.

You should focus on keeping dist to players that can hit you from distance and lasthit minions. Keeping adventage on minion fight area is key to pump character fast.

Most important is to save mana, cause till lvl 10 you will lack it a lot.

your mid game starts when you'll buy Executioner's Calling.
You can search for 2/3hp players and try to combo them.

Visiting jungle might be option for few gold pieces and xp points

piercing arrow - used in 2 cases:
1. enemy have low hp but runs away too fast to chase him
2. keep enemies away from you when you're weakened on line (abusing = instant lack of mana)

blighted quiver:
nobody really knows how strong he can be if used well. You should max it second.

Arrow storm:
max it first!
deals good dmg, cheap, slow enemies, deals deep wounds, used on minions gives you nice farm abilities

ulti:
used in late game for teamfights, in early game i use it usually as 'runaway button'

Normal combo
start with arrow storm then basic attack till you'll get storm back or enemy will start to run away, then arrow storm and deflective arrow.

Suprise combo (when you go on somebodys line)
start with ulti, then 2 regular shots, then arrow storm and (till in range) normal attack, (then) piercing arrow

Very important part of varus is knowlege of running away.

Running away when enemy is slighty faster than you:
-flash out
-throw arrow storm under yourself, he will be slowed when he'll walk on it

Running away when enemy is much faster than you:
-flash trough object
-use ulti on him. Ulti gives you 2 sec. so good option is to throw arrow storm when he'll be released

On teamfights you should be on background till opponents will pick targets, then you can enter with ultimate in the middle of fight and glue up arrow storm in the same spot then use base atacks.

If enemy second line don't have full hp good idea is to scary them abusing range of piercing arrow, this should cause caos in second line.

NOTE
You should avoid big fights till purchase runaans hurricane

upsides:
-carry useful in midgame
-good farming possibilities
-Strong combo and possibility to chase down opponent.
-stealing life + runaans hurricane gives you chance to win with stronger opponents
-strongest on line with chars with jump to enemy and strong dmg (like jax)

downsides:
-easy to be killed
-weak solo
-useless till (about) 9 lvl