Hi, I am a Riven player with 100k mastery, I also play akali and yasuo mainly. I made this guide with lots of experience over season 11, most of my riven games were played during this season, I am no expert but I know what I am doing.

Runic Blade is Riven's passive which gives your next auto attack bonus damage every time you cast an ability, stacking up to 3 auto attacks. You can abuse this by using the Broken Wings + AA + Broken Wings + AA + Broken Wings + AA.

Broken Wings is Riven's first ability in which she dashes in the direction she is facing or the target that you have your cursor in, dealing physical damage to anyone in range of the dash. This can be reactivated within the next 4 seconds. Broken Wings 2nd cast is equal to 1st cast.
Broken Wings 3rd cast is a bit longer than the 1st and 2nd, but knocks up enemies around her and has more range. This can also be used to jump over most of the walls in the game, known as "wall hops".

Ki Burst is Riven's second ability in which she stuns everyone around her for 0.75 seconds, dealing damage.

Valor is Riven's third ability in which she dashes towards the cursor direction, getting a shield for 1.5 seconds.

Blade of the Exile is Riven's ultimate ability in which she empowers her sword gaining (20% AD) bonus attack damage,75 range on auto attacks and some range on Ki Burst and Broken Wings for 15 seconds. Then she activates it again to cast Wind Slash doing damage in a cone where she casts it, lower hp enemies take more damage.

Doran's Blade Is the most common starting item for Riven when you are in matchups that you need to all-in or trade without getting damaged.
Doran's Shield Is a good start for Riven when you're laning against poke champions.
Long Sword Is a good item for Riven because it provides damage and sustain.
Ionian Boots of Lucidity Pretty much you want to get this as soon as possible as it gives you so much cdr for outplay potential. Take other boots if they have heavy ap/ad.
Goredrinker Is the most common item on Riven at the moment because it gives you CDR, healing and damage. Its the best mythic for Riven.
Black Cleaver Is the 3rd most common item built in Riven because it gives survivability, damage, movement speed and armor shred. I recommend taking it when you face tanks or when you're even/behind in lane.
Essence Reaver Is the item that you should get against squishys, this gives you a lot of burst damage and split push potential so it's better for snowballing.
Sterak's Gage Is one of the best 4th buy for Riven because it provides a lot of survivability especially in teamfights, around 4th item is when teamfights start, and it also gives you some damage. Steraks is the most buyed 4th item. Maw of Malmortius vs Heavy AP teams instead of steraks.
Death's Dance is the item that you pick if you still need survivability in team fights and the enemy has lots of AD.
Spirit Visage is the same as Death's Dance but vs lots of AP instead, it also gives you extra vamp and healing which is huge.
Ravenous Hydra Is the item you take if you feel that you need damage against squishy targets. Also, Ravenous Hydra gives you amazing wave clear and split push potential and a lot of omnivamp.
Serylda's Grudge Is the item you take if you need damage against heavy armor teams.
Chempunk Chainsword Is the item you take if they have a lot of healing AND you cannot one shot them before they heal.
Silvermere Dawn against champs that have certain CC you need to get out of. Example: Skarner, Malzahar, Mordekaiser.
Guardian Angel is for survivability or for when you are huge and you cannot die or you have a huge bounty.