Play defensive for the most of the time to keep the deaths as low as possible.
Last hitting is not really as inportant in normal games as in ranked but try get as much lasthitting as possible for early health and damage items and remember to join the teamfights.
Riven is needed in the teamfights cuz of her cc´s. The stun and the knockup really are great to help your team get some easy kills or save teammates. Also always start the teamfights activating your ult for extra damage and an easy kill pickup if enemies try to escape. Never use your ult unless your target got low health or if the duration is wearing off.
If you see that an enemy champion is going to charge you, just pretend your not seing what they are going to do and when they are close enough stun them with Ki Burst and shase them down with Broken Wings.
I use flat AD Marks and flat AD Quintessence´s for early game advandage and the flat armor Seals and flat magic reist Glyphs for extra defense and minimized health loss. What this does is giving you alot easier laning and bigger damage reduction. You can switch the flat AD marks to armor penetration if you want that instead.
I like to chose my skills even for higher damage and better damage absorb. If you miss with one skill you always got another one. Broken wings are a great skill for shasing down enemies and to get them knocked up so you can hit them with your Ki Burst, wich could result in an easy kill combined with Blade of the Exile´s ranged ability. A full combo of skills should look something like this in a normal fight when you attacks an enemy out of melee range: R Q Q Q W E R. While engaging a teamfight could look like this: R E Q Q W Q R. Also Broken Wings and Valor are really good when you need to escape from maybe a gank or if you get too low in a teamfight.
Use Ignite combined with Flash for easy escape and assasination mechanics. Heal and Exhaust is not a bad pick either. But i prefer Ignite for easier early game kills. If you want to take your role as the teams tank Clense could be something good to have too if your up against alot of cc champions in Draft pick mode or Ranked. Some summoner spells you should never pick going with this build is following: Promote,Clarity,Smite. I prefer not to take Teleport either because of the really long cd and the fact that you are playing tank/off tank means you will be on all lanes helping your team and always be on the move.
In ranked this build is great for solo top. Pick as many last hits as possible and use your Valor to charge at your enemies and stun them with Ki Burst for great harassment. Never forget to hit your target with auto attacks between skills for your passive to give you even more damage output. Also buying a ward or two to but in some bushes around to prevent incomming ganks or to just have an eye on the Dragon or Baron Nashor. In ranked play as tanky as possible and dont be too greedy since that can result in unnecessary deaths.
This build is made in a way that makes you deal high damage and still have enough health and resist to take the role as a tank in teamfights. Engaging the fight with Valor to give yourself more damage resist, and if thats not enough you got your Maw of Malmortius to prevent magic damage when you fall below 30% health.
When farming on riven you should always start with using all your stacks of Broken Wings and finish the minions with your Ki Burst. Also doing the opposite works too. Harass the enemy as much as possible if they get to close so that they know that if they try to hold you away from getting your last hits they may end up dead. Farming is something you are not supposed to do durning teamfight because that is just stupid and you wont get any kills or assists and your teammates needs your CC. So dont farm durning teamfights. Remember that the early farming ahve the most inportant role in the game and that the late farming dont do quite as much difference.
Creeping / Jungling
On larger monsters in the jungle, make sure to have a auto attack between every skill ussage for massive damage. Sometimes going in to the jungle will give you faster items. This rune setup works for all AD champions in the game that are capable of jungling. Never take the friendly jungle buffs unless your teams jungler says its ok. Stealing the buffs from the enemy jungle or just taking the big Wraths or Golem as counter jungling is really great and can totally ruin the enemy junglers jungling for a moment. That will result in that the enemy jungler will be under leveled and wont be able to do any good ganks. And help your jungler to sometimes look out for enemy counter jungling and try stop them if possible.
This is my simple Riven guide. This is how i play Riven and i hope that this guide helped you playing Riven as a offtank or tanky dps carry as i do. If there is anything you may want to ask about the item setup skill setup or my masteries, then just leave a comment bellow the guide so i can hopefully improve this guide and make it better and easier to understand.