It may be true that this is my first build guide, and that this may seem like a stupid idea, 40%cdr varus is a great way for beginners to learn adc. By Simply maxing your q damage and cdr, you can hide in the back line and spam q, the enemy can't hit you sand your arrows hit like a truck. (I'll try get images later).

http://matchhistory.oce.leagueoflegends.com/en/#match-details/OC1/79976769/200204454?tab=stats
Please note: You dan't have to be too fed to carry the game

Pros

Q op
Q has very long range so you take no damage
Q can kill a tank in 3 hits
Q op
Q is great early game
Q op
Can do high amounts of damage even when behind
Q op
Ulti is great cc
Q op
Can q every 4 seconds
Q op
Did I mention q is op

Cons

Slow (I guess)
Requires the ability to hit the q
Hard to cs early (who cares as you get early kills cause q op)
Q IS TOO OP

I get flash and heal as these help me escape and disengage if I get caught out (you shouldn't if you ward properly and stay in the backline)
Other viable spells

Ghost: You can replace heal for this. The movespeed will last longer than heal, but in my opinion you only need the bonus movespeed for a short time and heal also heals you.

Clarity: People think it's bad (because it mainly is) but to start off you will probably miss a lot of q's and this varus build can be very mana hungry

Exhaust: It is ok for disengaging, but you will have to be pretty close to do this, so I think heal and flash is still better.
AVOID:

Ignite: Don't get me wrong, ignite is a great spell, but you will be too far away to activate, and you finish off people with your q anyway.

Smite: Why would you use this if you are not jungle, if you want it to be a good item, it requires an extra item which can have a better use for more ad, and you will be out of range as well.

You will have a lot of trouble farming with low attack speed. You will want to spam q and if you can hit minions with q as well do it (your q will do less damage to targets behind it and you can use this to make your opponents underestimate your damage). This will get you a bit of gold, and use your high damage to kill creeps then run away.