i actually main yasuo in mid lane and items like shiv and infinity edge are his must haves. on the trinity force thing his Q acts like an auto attack which some people don't understand. His Q will actually activate the spellblade and consume it making your Q do double damage on top of a crit. Also another thing to note is that since IE got nerfed with it's crit chance being reduced yasuo doesn't have as high of crit rate with shiv and IE alone and Trinity Force gives you an extra 10% X 2 because of his passive which makes up for it.Another good thing to build on yasuo is a life steal item, between bloodthrister and hydra are the two biggest options. Take bloodthrister if your getting a lot of 1v1 fight as it will get you more lifesteal, but as far as team fights go hydra will absolutely chunk people in an area. Hydra also causes your ult to do damage to people around where you are ulting even if you didnt knock them up.
Your is a fixed distance dash in the direction of the target so just keep that in mid especially when your close to minions when you dash to either last hit or just for movement/harass.
a really agressive build i like to go with is
static shiv
then Infinity Edge
then a Bloodthrister or Hydra
a Trinity Force
and a Last Whisper (90% armor pen off ult combined)
(as far as boots mercs(against AP), beserkers(if ahead) or ninja tabi(against AD) are fine feel free to buy those whenever you have the money)
some people like randuins and Guardian Angel on Yasuo and i used to as well until i started using bloodthrister often as i would heal so much off my Q and auto attacks
As far as ulting people dont be afraid to go in. The only way you are going to learn the damage you can do is by going in and fighting. Worst comes to worse you end up dying. Oh well. But i find when i have at least some lifesteal i end up winning a lot of fights i didn't even think i would survive. A good thing to keep in mind is that you can harass and deny farm by just holding your ult and send out your knock up whenever you can. Keep in mind you have a 2 second window to decide to ult or not so keep that in mind as your able to Q or AA as their knocked up before you ult in if your close.(A nice thing to note if you Ult, if you knock someone up in tower range your ult will try it;s best to position you out of tower range if it can, tower also can't hit you while ult is going off) In team fights try to ult early to get the 50% armor pen early so you can chunk people's HP even quicker despite their armor.
Best of luck playing as Yasuo as he is probably my favorite champ and the funnest to play. My best advise would keep playing as him and practicing him as he is hard to master but is very rewarding when you do. Feel free to watch pros like Bjergsen on TSM, Xiaoweixiao on LMQ, Froggen on Alliance ,and even Faker on SKT T1 as they are all amazing yasuo players.
