There are four builds here.
I want to note that although I put the last whisper in a lot of these builds, often times I'll notice the enemy team hasn't gotten armor, in which case I don't get it. But because I don't grab armor pen marks I grab this if they do grab armor.

When I go back to the shop, I buy whatever I can afford in my build. If I don't have enough for a B.F. sword and it's the next step, I don't just not spend my 1200 gold, I buy stuff. This short term advantage can get you that kill, and totally be worth it, plus, you don't want to be going back to shop after saving up 200 gold, because then you lose xp and gold from going back so frequently.

1: AD Varus Standard

2: AD Varus Fat Enemies

3: AD Varus sustain lane

4: AD Varus with a Soraka support

5: AD Varus mid

I use this build when I don't think I need that extra sustain with boots pots. I feel this way when the enemy support isn't Soraka, and when my support has heal as well, or has a heal spell.
Also if you are against a bot lane that isn't as strong as you early game. For example, if I want to shut a vayne down early.

If you notice the enemy team starts to have a lot of HP, including players you wouldn't expect, like their support or ad carry, I added this build because you'll want to grab a Madreds.

If I don't want to worry about getting pushed back to my tower early, and I'm not feeling like my lane is going to be able to be extremely aggressive, I'll start with this so that I have some room to take some hits. With the AD runes you'll still be in a good place to harass.

I made this because having a Soraka support changes your gameplay. She is an awesome support that is depending on you to do damage. She'll give you mana and hp, and you better be dishing out damage. The exhaust is taken because you don't need the heal, and because you'll get that kill faster, because you'll be in control of the entire initiate.

I don't mind playing Varus mid, and sometimes my team will request this. This is usually in player matches rather than ranked, but just in case you want to try it out, here it is. I'd still take note to the other builds because you'll learn what to do if the enemy team gets a lot of hp, armor, etc. All these builds combined can teach you a lot and allow you to change your build on the fly.

Pros:
Great early harass with Q if you can hit it (laned with Nidalee, omg the ranged harass :D)
Great farming
Has a slow and ulti stun so some escapability
Good damage all game
When you do your ultimate well, your team has a huge positioning advantage

Cons:
If you can't land that Q, it's gonna be rough

You are the ad carry, it's time for you to do tons of damage.
What's nice is that even if you aren't the fed one on your team,
or you're lacking in damage/skill with Varus yet, you are still
a good asset for your team. Your ultimate and your slow allow
you to be helpful even when you aren't destroying the enemy.

Set the ultimate off when the enemies are close together, they'll
pass the stun off to one another and you'll get a huge positioning
advantage.

Your Q!!
It is important that you charge your Q up to the max as much as possible,
the damage is increased and it's worth it even if the enemy is close to you.
Don't do it if the enemy is a burst from death.

Get your Q first.
It charged to the max at level one does a nice chunk of damage. I start harassing immediately.
This is why I get my slow second, so I can get a few extra auto attacks in. I find the extra
auto attacks plus the damage from the E is better than getting my W second.

I go with flat AD runes for Marks and Quints because the early damage works great and is still nice at the end. I get good harass early, and even with boots pots do well. It also allows me to go boots, pots, vamp scepter, veal, and still be doing damage.

Obviously the rest for defense.

I take the bit in utility because he is mana hungry. I do however harass a lot in the beginning. I pretty much spam my Q if I'm doing real well and they're under the turret. The regen allows me to sit back for a little and still be able to use all my spells in case of emergency. The amount of damage masteries I use also reflects my aggressive damage output play-style.

Every game can and should be analyzed as you play, changing how you build. Maybe you buy a wriggles to get that armor rather than going immediately for the B. F. sword if you're getting hit hard. Maybe you don't need the Last Whisper because you're team is AP heavy so the enemy isn't building armor. Maybe you require a bloodthirster before an infinity edge. Perhaps the Madreds is necessary. Each game is different, play it that way. Understand your items, even though I've made a bunch of builds so you don't have to understand the items, you can just choose which build looks like your game the most.

This changes based on the fight.
Use Q to harass, the damage is nice and the range is great.
In team fights if you're the only CC you're going to have to use it early.
However, usually you won't be the only CC because you aren't the initiator.
Wait for your team to use the CC, then pop your ulti mid team fight, this way
you guys maintain an advantage.
I usually pop my Q immediately, then follow it with an E to slow everyone I can.
Keep auto attacking the squishies, and use that ultimate when you need to either get a melee off of you, or the enemy is close together, because the stun passes to other targets. If you can get them all stunned, you'll have a nice advantage on focusing targets.

I use heal flash. Flash for survivability, heal for the same. Heal is really nice because someone will jump on you and think they've got you, then you pop heal and finish them. You can make fights you know you'll win happen, because they think they'll win.

I use exhaust instead of heal when I'm laning with someone with a lot of heals, like Soraka for example.

Try him out, don't get discouraged because he's not the easiest champion to learn. If you can master his Q, you'll do just fine.