Please let me know what you think of this, it's my first guide...

This build might strike some as very simple but that's the real beauty of Riven really, simple but effective and with some more complex nuances when you get to know how she works.

This build aims to utilise Riven's role as tanky dps; I doubt much of this guide will surprise people but that isn't the point, this guide aims to build Riven as an effective off-tank/dps. If you want quirky, complex or alternative, you might want to look elsewhere.

Riven is a viable solo-top but this build doesn't cover for that, this is meant for a duo-lane set-up and can work alongside a carry or support very well.

Riven is good at performing bush-ganks thanks to her Valor + Ki Burst.

Riven's Broken Wings ability should NOT be spammed (unless you're using it to run away), instead allow an auto-attack between uses to get the maximum benefit out of her passive.

Rune choices for Riven are many, my experimentation has led me to value defensive-oriented runes and offensive items, building defensive items later in the game.
Marks
Greater Mark of Desolation x9 is pretty much a given for a 100% melee champ
Seals
Greater Seal of Armor x9 will give you enough armour (esp. when added to Masteries) to last until mid-game (usually)
Glyphs
Greater Glyph of Magic Resist x9 gives enough MR to last until mid-game (usually)
Quintessences
Greater Quintessence of Health x3 finishes off our anti-squishy-Riven campaign. Congrats, you now have one of the toughest early game champs.

Viable alternative rune choices

Many other builds would recommend the following, I'm recommending the above as it fits in with the rest of this build (masteries & purchase order are the way they are because I'm assuming you have the all-new non-squishy Riven) but the following are also good choices for Riven.

Greater Quintessence of Vigor very useful for those games where you've bought your starting items then find yourself up against high harrass.
Greater Quintessence of Desolation for a high dps build
Greater Glyph of Cooldown Reduction as Riven's only restricting factor on ability use is Cooldowns, these runes are very powerful but item build & already-short CDs make them non-essential.

Riven's Masteries should be fairly self-explanatory but for those who like it spelled out;
Deadliness 3/3: We're not using Ability Power or Smite, kinda narrows our choices down on this tier, will be getting more crit later with items.
Cripple 1/1: We will be using Exhaust so why not buff it?
Sorcery 4/4: Riven's only restricting factor to abilities are CDs so this means more pewpew more often!
Alacrity 4/4: We attack so attack speed is nice, not essential though, can be semi-swapped out for 2/2 Offensive Mastery if you want to hit those pesky minions a bit harder.
Brute Force 3/3: Extra damage, nice :)
Lethality 3/3: This is what we came here for really...
Havoc 1/1: We've got this far in the tree which makes this last choice a no-brainer.

Resistance 3/3: This supplements our runes and goes towards making Riven very tough early-game
Hardiness 3/3: See above (Resistance)
Evasion 1/4: A spare point, put this where you want, having some dodge didn't seem like a bad thing.

Haste 1/1: I prefer to use Ghost with Riven, others prefer Flash but the buff to flash isn't attainable with these masteries so you'd need to find somewhere else to spend this.

The above item build should be followed, as always though, item builds should be fluid and responsive to your own team's abilities and your opposition's make-up. With this in mid, consider the following recommendations;

Riven may struggle to farm & get kills vs a good ranged champ (Kog'Maw, Ashe & Caitlyn are tough lane opponents), if you find yourself against one of these, you may wish to get Heart of Gold and/or Regrowth Pendant as soon as possible to help with the ensuing gold & HP drought.

Guardian Angel is in this build as a placeholder. It is a good item if you are facing a balanced team but should be substituted for a Force of Nature or a Glacial Shroud to counter the other team's carries. Sunfire Aegis is also an excellent item as you're going to be in the thick of it often later.

If your team already has lots of slows etc. then substitute Frozen Mallet out and go straight to Warmog's Armor and add an Atma's Impaler later, sooner if there is a lot of physical damage on the other team.

Substitute Ionian Boots of Lucidity for Mercury's Treads when facing a heavy CC team, other tenacity items are poor for Riven.

I've listed Riven's abilities below with all tips I know relating to that ability appended beneath it.

Passive Runic Blade
Learning this is the key to being a good Riven, where possible allow an auto-attack between your ability uses to make use of this. Also see how it should affect your item choices, this is why Riven shouldn't go for AS items.
Q Broken Wings
AoE ability, great for clearing minion waves. Excellent for spiky damage on enemy champs. Weave it's use between your auto attacks to get the most out of Runic Blade. The 3rd use works like a mini-flash, you will jump towards your cursor so try to hit ahead of fleeing enemy champions to knock them back.
We don't level this skill first because it's cooldown doesn't change with subsequent levels.
Spam this with your cursor in front of your champion if you're trying to escape to add to Valor.
W Ki Burst
Use to interrupt enemy channeled casts e.g. Malzahar, Nunu, Warwick, Katarina, Miss Fortune.
Level this first as it does good damage and cooldown decreases with improved level versions.
Useful for clearing minions.
Use after using Valor to get in position (remember to try getting in an auto-attack first to use a charge of Runic Blade
E Valor
Use to initiate, escape, gain a shield buffer or even just to generate a charge for Runic Blade
You cannot cross over walls using this.
R Blade of the Exile
Save for fights where you will be going for the kill or vs >1 enemy champion.
Use this ability first before performing your normal attack cycles.
Save the Wind Slash use of this for champions on low-mid health.

Credit to Wafflator for his excellent Riven build for the following excerpt:

Exhaust
Excellent for 3 reasons;
Chasing
Running
Supressing high damage champs

Ghost or Flash
For chasing/escaping

Others are viable but these are the most notably useful.

Just don't, for the love of God take Revive, Clarity or Rally!