Edit: added a different skill sequence that prioritizes Valor and Ki Burst on second slot

NOTE: This is my first build so I'd appreciate it if people would help instead of down-voting or criticizing it.
So I recently got Riven and due to the lack of guides (or at least the sheer number of guides floating around) I decided to try making my own.

Looking at the descriptions of Riven, one would think she'd play like Tryndamere (both AD champions with no cost for abilities). But personally, I feel that they're two VERY different champions. Whereas Tryndamere benefits the most from his critical and attack speed, Riven is more focused on using skills and auto attacks (a bit like Lee Sin)

Edit: Thanks to mastajdog on the rune suggestions, ended up balancing out quite nicely

Bear with me on these, mainly because I seldom use runes. Any suggestions would be appreciated.

Building with flat Attack Damage Marks, helps early game and benefits her skills
Flat Armor Seals for added survivability
Flat Cooldown Reduction Glyphs (cooldown based champion)
and Armor Penetration Quints

With the mastery tree I'm still trying out different ones, but the one that I'm trying to use now is a 21/9/0 (again, trying to balance out attack and defense). Like the runes, help would be appreciated.

I usually take Ignite and Ghost as my summoner spells.
Ignite of course to finish off opponents/prevent them from healing
Ghost to either hurry back to my lane or close distances.

Possible alternatives would include: Exhaust (never underestimate a well timed exhaust), Teleport (handy for map control), and the all time favorite Flash

I typically start out with Doran's Blade, which i sell later on if I need to. Mercury's Treads are, in my opinion, the best boots for Riven because her Broken Wings and Valor both allow her to cover distances pretty well. Other than the shield she gains from Valor and the stun from Ki Burst (half second stun regardless of leveling), she has no defense other than her movement (basically: if she can't dash around she's dead) With Merc's, I can usually outlast most cc's (unless they're ridiculously long like Ryze's) after which I dash out as quickly as possible. As you can see from my items, I focus on auto attacking more. I'll explain this later on in skills.

Edit: Added alternate item build on second slot, still testing out (might need some advice)

Runic Blade (Passive)
Riven's passive, it gives her a charge which increases her damage on the next auto attack. She stores up to 3 stacks at any one time and has a time limit to use the stacks, after which they disappear (auto attacking refreshes the time limit on any remaining charges).
This is why it's vital to increase Riven's attack speed to a decent amount and combo because without it, you lose a large chunk of damage that you can dish out. It's extremely aggravating when focusing on someone and not auto attacking, congratulations you've just wasted your charges and your target gets away with just enough hp to survive your ignite (yes it's happened).

(Q)
Max this out first! This is arguably the MOST important ability for Riven, each activation counts for one charge on her passive and it can be chained within a time limit with an almost instant cooldown between the three activations (13 second cooldown regardless of level - starts after FIRST activation). What you would want to do when playing Riven is to use it, auto attack, repeat.

Q - right click - Q - right click - Q - right click

It's also a very convenient way to move around/close distances as each cast moves her forward by a small amount. How the dash works is that, if your cursor is over a target or target area, she dashes in that direction while not specifying where to dash will result in her dashing in the direction she's facing.
NOTE: THIS IS NOT LIKE TRYNDAMERE'S SPIN, IT CANNOT PASS THROUGH WALLS

The second skill sequence I tried involved maxing this out mainly because the damage scales less than Ki Burst and the cooldown remains the same throughout, one point in this will be enough if you want to focus on her other abilities.

(W)
Most of the time I max this last for two reasons: 1) I focus on higher mobility through Broken Wings and Valor, and 2) While the damage does increase per level, the stun duration does not.
What you want to do with this skill is to dash in with either Broken Wings or
Valor (Valor if you want to avoid some damage and Broken Wings if you want to try and combo) and IMMEDIATELY stun, half a second may not be much but it can make quite a difference in teamfights (i.e. stunning Nunu when he tries his ult, stunning the Carry to take them down faster, or even stunning the tank to prevent cc) the possibilities are quite numerous.

With the other skill sequence I upgrade this and Valor at the same time mainly for the cooldowns and AD scaling.

(E)
One of my favorite skills, I've lost track of how many times it's saved me/helped me get back in my lane. What it does is, like Broken Wings, Riven dashes forward but this time she gains a shield that scales with AD (For an AD champ that's pretty convenient, closing distances, gaining a charge, AND soaking up a fair amount of damage). Like Broken Wings though, it can't pass through walls. I max this out second in order to lower the cooldown and basically dash as much as I want.

Same as Ki Burst

(R, Ultimate)
A two-part ultimate skill, Riven restores her sword, increasing her attack damage and range. During the time it's active, she can reactivate the skill to use Wind Slash (A large cone shockwave in one direction that scales with her attack damage AND a percent of the target's missing health. This'll usually be enough to finish off most opponents unless you initiate in which case you beat them to death with your ridiculously over sized sword.

So in a nutshell, this is my guide for Riven, the Exile xD
Feel free to rate, comment, or make suggestions (please, help would be really appreciated)