You should start early by showing some love to your jungler and help him with buff if needed.

On the lane, you should try to poke a lot. Use your Q for your advantage but keep in mind your mana. You can usually go for some form of mana refunds with your E but it can get risky. Your W is not very strong currently but you can use it when you wanna outpush the lane asap but poking with it is pretty much pointless currently.

Your E is for free but have long cooldown so try to use it only on stuns or roots. If enemy dont have that, you can use it to prevent some poke (like Karma's Q or so) but overall, dont use it too often since if you use it to something silly (like Ezreal's Q) you will really miss it right after when you will see Blitz's hook flying at you. Try to farm more efficient, with keeping all enemy minions healt relatively same and then just took them all with Q so you will also poke your enemy.

After finishing your Mythic, you can start playing more agressively and take as many turret plates as you can. However, you have no dashes so save your flash for junglers/roamers and rather escape from enemy via your ult and E, if its possible.

Dont forget to show your love to jungler while taking dragon or fighting in bottom side river (1 Q may get you at least assist, thats worth leaving line for 15 secs)

Sooo, at least for me, Mid game starts when first towers starts falling, so big advice first, use your W on tower, so you will take tower and minions as well. Besides that, keep farming, take dragons and push lanes whenever possible.

Sivir ult and Q is made for teamfights and you can deal tons of damage + increase DPS and mobility of your team-mates, use your ult as often as possible, just dont forget that you dont have dashes so dont get yourself caught by some jarvan ult or so (even your E wont save you there)

Your W is quite good for taking enemy Raptors and dont mind spending 1 Q to get even other camps, dont be too brave tho and just keep yourself in safely close nearby your team, but still, try to take as much enemy jungle as its safe for you, thats how your team will snowball and win even faster.

If you are just KDA hunter, save your Q for when enemies will be low and then just take kills with it, if you are more interested in climbing and more wining, I recommend you to use Q even for some poke, with esence reaver, you will get lot mana back and later on, your cooldowns on it will be smaller anyway (especially with reaver) many sivir players dont focus midlaners in teamfights and go for junglers cause they are bigger threat to you, but midlaners are bigger threat to your team, so i recomend to focus midlaners instead (only if you cant go for enemy ADC ofc)

Push Push Push! In late game, most ADC's will have higher direct damage than you, tho, you have tons of damage to multiple targets. So basically, plan is: win teamfight, and rush for enemy nexus. Always be close to your team and never go alone otherwise, even with your E, you will get killed quite easly in late game.

Dont forget to keep stealing enemy jungle, especially buffs. Try to get a baron and rush for finish. Also, in long term siege, your huge AoI is great advantage so however it is, try to be constantly knocking at their doors.

No team will win without pushing towers/inhibs/nexus so mind that and dont get distracted by mindless kill hunt, it will only raise your bounty so if you will die, your enemies may get a chance