Varus is the carry that counter the other carries.
I got nothing else to say.

You should try to get as many last hits as you can, even though if they might make a hit on you, Just don't die. Try to harrash the enemies with your Q ( hide in the bush and charge it ).
Try to do as much possible damage on the minions if your enemies are offensive. Just don't push the lane.

Well. Items depends who you're up against.
If you're against like 3+ AP, I' would recommend to buy Maw of Malmortius instead of the last bloodthirster. This will gain you quite some bit magic resist, and also give you tons of extra attack damage.

Try to make the items fit the enemy team, so you easily can counter them.

( Oh, and I'm aware that the build i've made isn't in order.. Try to make it fit with the money you have at the time you buy.. I recommend an early BF, before boots if possible.

As runes I've choosen 3 quintessence which gives you 6 percent lifesteal total. I prefer this over damage runes, since you might have to hold your lane for quite some time, before considering going back and buy. This will also give you the oppotunity to buy dorans blade, instead of boots and health pots.. You will start out with 9 percent lifesteal early game, and I bet the enemies wonder how you gained the life back you just lost ;-)

For marks I've choosen 6 x 0,95 damage, for some tiny extra damage.
I've also decided to put in 3 x 1.66 armor penetration, to gain penetrate a small amount of their early game armor.

For Seals I've choosen 9 x 1,41 armor, since Varus can be a little squishy early game.
That's why I've choosen 9 x 1,34 magic resist too.

You can try to play a little with this, and pick some other runes, this is just what I thought would be the best.

I've choosen exhaust as my recommended summoner spell, due to the armor reduction. You'll realise that i'll become very handy, in nearly all situations.

As my second summoner spell i've choosen flash. Flash is the way of escaping, and get the last hit in on escaping champions..

You could also try to exchange exhaust with ignite, since ignite just will lower their health regen, and deal true damage.. Very easy kills, if used well..