First off machine gunning is AWESOME with varus for 1 he gets more attack damage withand he attacks faster when he kills a minion or a enemy champ! not only that he also stacks blights that explode upon the use of his other skills but not just that he can slow his enemies rendering them helpless with his not to mention he also has a great reach skills to shoot past those annoying turrets for the final hit

(hes also a mean mean kser with his speed and sneaky attacks)

ALL THIS IN MIND HERE IS MY GUIDE

First I feel I should start with your skills.
I would start with this. Its going to help you a lot with getting lanes pushed giving you more damage per hit, not to mention its magic damage. It also gives you a chance to stack enemy champions with blight so later when you get skills you can use them right off to full use. remember they only stack 3 times so try to fit your "HAIL OF ARROWS" in there between every 3 shots.

I would get this next its useful to varus in two ways
1. You can use it at the beginning of your attacks against an enemy champion to slow them down a lot as well as damage them a lot, this will help a lot later when you are shooting around 2 arrows a second and they can't get away fast enough

.2 you can use this to slow an enemy that is running away from you by placing it in there way
this will help you machine gun them to death or even launch a Piercing arrow at full draw landing the final shot.

this skill is your 3rd most important skill, I say this because really at mid game you should be able to machine gun your enemy into ground champion without this skill, but if the enemy is just past there turret and you can't follow them for the last machine gun round for the kill this arrow is gonna land it for you.

Last but not least

this skill is a must have when your going 1v1 if the enemy is squishy hit them with this and then spam them with arrows followed by "HAIL OF ARROWS" by the time they get out of there chains they will be bellow half health and a single will finish it up

If the enemy is tanky I recomend giving them the run and shoot treatment until they are down to at least half health then catch them in chains follow the same order as squishy at this point "HAIL OF ARROWS" followed by machine gun arrows and a ftw.

The only two I would ever use here will be.simply because you can heal when your hp gets low. you can not beat this especially when you are using life stealing stuff at the same time. also your teammates will thank you

and

this one is easy Varus has no retreat skills when a gank is coming his way so flash is the only option to gtfo before your ganked. no one wants to feed the enemy. NO ONE

Varus plays well with people who can stun slow or pull back enemies to him. he doesn't play well with clerics or ksers like teemo.

List of good allies:
with his arm pull back this is the best choice.
with her slowing arrows she is also a good choice as well well as her stun ulti.
Also good with her web attack and her ability to pick up kills, assists help too you know?

so far these are the ones I would like to see up top with me alistar maybe but no one plays him sadly.

This is easy with your life steal items you will easily be able to push a lane alone if there is no enemy champions in your lane even without your minions helping you. You will easily be able to stay alive mid game. until then your best bet will be to just spam your arrows at them and maybe use your "HAIL OF ARROWS" but I would save that skill for if you see a enemy champion.
you will attack fast enough that you will be able to run minions into coin super quick as well as turrets.

Shut up and stay in your own lane.

if you ever want to jungle I recommend shooting your skills over bushes and trees the enemy will never see it coming.

If you are gonna try to kill mobs in jungle don't bother you're leaving yourself open for ganks and ganks are varus worst enemy.