This is just a basic guide of what you will be most likely wanting to buy with this new champion.
I have played a lot of games with her already, and I must say this champion is awesome if played correctly.

Riven is a melee champion, intended for damage.
She also stays as a damage champion.
If you even try to tank with her, you will be finding yourself losing a lot of good stuff.
Her Valor shield is based on attack damage as well, so you really will be wanting to get some of that goodie attack damage.
Also as the only thing limiting her attacks are the cooldowns, you will be running after some CDR as well.

Ability Power and mana are useless on her as well.

Pros:
- Lots of damage from charging after enemies
- All of her skills are based on AD, so you can gather a lot of normal weapon damage as well.
- AoE ultimate with fairly long range, which also gives bonus AD.
- AoE stun to interrupt those pesky channeler spells.
- Lots of jumping around, you can really catch those enemies.

Cons:
- Weak survival, will need the help of teammates often if vs. a ranged with lots of health.
- Ultimate is really hard to shoot at persons while in close combat, as it can be side stepped easily if you are unlucky with timing.

A lot of players will probably disagree with taking Valor first.
It is still a note worthy ability, as you can charge away with it and absorb some damage, so it will really save you from those nasty ranged attackers and ganks at the start.
As your stun has a really low range, you should get Valor before it.

You will be wanting to go for a lot of AD due to that being the only thing that helps her in damage.
For survival, you will be wanting a lot of lifesteal, this way you won't be losing that much in damage.
Of course, if the enemy team is full of melee, you should get some armor as well.
Shen and Trinity Force are useless on her, you will be gaining a lot more benefit off of pure AD.

After you have gotten all 6 slots filled up, sell Executioner's Calling.
Note: The only reason why you'd want to have 2x Infinity Edges, is that you have run out of item slots but you have tons of gold to spend. At that point, sell the Executioner's Calling and get another Infinity Edge for 50% critical chance and an additional +80 damage.
I suggest getting 2x the Bloodthirster's, as their passives can be stacked and if you won't be dying a lot, you will find them really useful.

Optional:
You may also want to consider a Frozen Mallet instead of the Black Cleaver, if your enemy team has a lot of caster champions.
This would allow you to chase them down and as casters rarely have high amounts of armor, this is a viable option.

For the runes, I chose armor penetration for some extra damage.

Although I said defense items are useless, yes, those are just items.
You should go for some armor on your seals, because let's face it, she sucks when it comes to defense.

Same as above, but magic resistance on glyphs.

For Quintessences, I took 3x cooldown reduction runes. This is so you will be able to use those skills of your a lot faster.
Attack damage per level runes are also fairly good.

Also, if you swap around runes, only take per level runes, as they will be giving you a greater benefit end game, and as your champion is already quite weak at lower levels, you will be wanting to buff that end game damage of hers.

Broken Wings + Valor harass

You can harass your enemies a lot with Broken Wings and Valor combined.
You charge to the enemy by running, you use 2 Broken Wings to deal some damage to the enemy champion / minions, if you see the enemy champion getting closer, use your Broken Wings once again to back away and use Valor to get even further away if needed.
If the enemy got scared of you doing this, just use your 3rd Broken Wing on whatever creeps you are fighting on the lane, to deal some extra damage. If the enemy starts moving towards you at that moment, run behind the creeps and THEN use Valor, do not use it if creeps are on the way.

Valor + Ki Burst + Broken Wings initiator

This is for initiating a fight, don't use this if you just want to harass the enemy a little bit.
You use your Valor and charge at the enemy, when he gets close enough, you use your Ki Burst to stun him, and to get some protection for yourself.
After that, you use your Broken Wings once, if the enemy stands still, use it for another time after a moment, if he still stands still, use it again.
If the enemy starts moving, then use Broken Wings asap to get in front of him, so your target won't be running away too easily.
Then try to keep up with the enemy all the time, use Ki Burst when close enough, preferably after you have charged to him.

[h2]Blade of the Exile[/h2]
This is a nice ability, but it's also a huge pain to get right.
It grants your attacks increased range and increases your attack damage by 20%.
Also, it allows you to shoot a fun AoE cone ultimate, which deals more damage depending on how low the enemy champion's health is.
You have 2 choices when to use this.
Either you use it when you are next to the target at the beginning of a fight, or you can use it at the end of a fight to shoot the ultimate and finish off the enemy.