This guide is what i prefer playing as Riven mid. I found this working out for me but i can't guarantee it will for you but it might so try it ;)
This is my first guide so be warned it might not be the best layout and written perfectly.

I would not recommend doing this if you are not comfortable with your matchup.

Longsword and 3 pots are going to give you quite good sustain. You could start E if you feel like you´re going to get bullied in lane. Longsword works well with your E aswell to get a bigger shield.

Because you're running sudden impact you don't really need to rush Dusk Blade. Getting Hammer will allow you to spam more abilities. Getting Mobility boots will synergizes with Celarity since its a free longsword for about the first 1-2 sec of a fight, this is good for and ganks and pokes straight after coming back to lane.

Youmuu's will give you more roaming potential and is why i prefer rushing it. Go Duskblade if you don't feel like roaming.

For squishy matchup i like going DD for sustain, but if you feel like their tanks are going to be a threat go TBC.

You could swap out any of the items for any of the suggested items. Duskblade is good for ganking to spot wards but red-wards works aswell.

GA is good if you are fed and can't afford to die and good against AD Assassins. Maw is good and might be a item you have to swap out if you are laneing against a AP champ with high dmg. Since the other items in this guide are quite important i would just build Hex first and finish it of later. Sterak's is quite a good item for bruisers atm and will provide good dmg and also keep you tanky. If the enemy team has a lot of AD dmg and you die to quickly i would go for Dead Mans.

I would mostly start rush R>Q>E>W, but if you are getting hard-bullied R>E>Q>W works aswell. but i don't recommend it.

Push! Push and PUSH! you mostly always want to push and poke with electrocute. Keep an eye out for their jungler and shove it under turret to deny farm. Getting lvl 2 and 3 early is strong and you want to abuse it. If you see an all in i would go for it (High risk high reward playstyle). This will also allow you to roam when you are pushed and spotting the enemy jungler is a good thing to try to do for a early kill, try getting your jungler to help out if you cant 1v1 with being higher lvls. Try looking out to gank bot or top after you finished Mobility boots with will snowball you even further.

If you cant win lane and abuse your laner with kills try roaming after pushing and look for scuttle or their jungler. Try going to the same side your jungler is at.