learn irelia tricks and combos
https://www.youtube.com/watch?v=0KmpoCl_hdg

Irelia's strengths

Irelia's strengths are that she is pretty safe in lane and she fits dive comps extremely well. Irelia doesn't fit peel comps because she is meant to dive people with her short cooldown Bladesurge and her ability to destroy squishy champions. So Irelia goes best with any team comp that can allow her to do her job. Champions like Orianna, Jarvan IV, Rek'Sai and Vayne go well with her because they allow her to dive or they capitalize off her dive potentional. Irelia doesn't go well with comps with a lot of single target damage, because she lacks AoE damage and with single target damage comps, they will most likely end up killing only one champion and have nothing left for the rest. In conclusion, Irelia best fits comps that have AoE damage and not as much single target damage since that is what she will be providing.

Irelia's weaknesses

Irelia is weak vs teams with a lot of revive abilities ( Aatrox, Anivia, Zilean) because she is supposed to assassinate carries, and will be useless if the target she assassinates can revive. She also does poorly vs teams comps with a lot of knock-ups because knock-ups cancel her Q. Irelia is also weak vs team comps with a lot of peel, because they can prevent her from doing her job. In conclusion, Irelia is weak vs team comps that don't let her do her job.

Irelia should always take Flash and Teleport. Her early game isn't strong enough for her to take Ignite and try to kill her opponent early. Having Teleport allows you her to safely get to her later levels without falling behind. Flash makes for her lack of mobility in team fights and is necessary because she does not have a reliable way of getting out of ganks as her Bladesurge needs to have a target positioned on her escape route.

Corrupting Potion is what you'll want to start most often.It gives you good sustain early, some bonus damage in trades and you don't need to buy potions anymore.
If you don't like Corrupting Potion and you feel confident you can go with Doran's Blade. Also, if you get Doran's Blade you can always get one more as it makes you very strong in trades.

Trinity Force is the strongest item you can get on Irelia. For many reasons. The number one reason is that with your ultimate you can proc Sheen multiple times which let's you deal tons of damage. It also applies Sheen on your Bladesurge which helps you oneshot caster minions for a free dash. The attack speed is also great on Irelia because she is very auto attack reliant. The extra attack damage and health from Phage plus it's passive is just a bonus that makes Trinity Force even stronger.

Every champion in the game needs boots. Irelia only really has 2 options for boots, those being Ninja Tabi and Mercury's Treads. If you are facing a team with mostly AD damage and little to no CC always buy Ninja Tabi. Vs most AD lane bullies such as Riven, Renekton and Wukong you will want to get Ninja Tabi pretty early to avoid getting killed in lane and getting set behind.

The second option for boots you can get on Irelia is Mercury's Treads. They give you magic resist and tenacity. You will want to get them vs teams with more ability power damage or a lot of crowd control. They pair well with Irelia's passive Ionian Fervor and/or Legend: Tenacity.