I am BattlEyeet, and I am a solid Silver 3 player (and not counting).
I play Sivir sometimes, but I believe I am an expert in the fast.

Pro: You go fast

Con: You do no damage

Anyone who can make you go zoom is a great pick. Enemies who slow will bring you down fast. Also hard engage/cc supports like naut, leona, etc. are really good for Sivir because she can zoom around them and kite them and dodge their skillshots.

First there's Phase Rush rune page:
This is the ideal rune page since you want to either proc it with
1. q (forward + back) + AA
2. q (forward / back) + AA + W (reset AA)
3. any other combination of 3 hits, but those two are fastest
Phase Rush gives you a lot of speed, so you can zoom a lot, good for chasing or running.

Then 2nd rune page is Fleet Footwork:
You do not go nearly as fast, but it helps with sustain is rough early games. Another benefit of this is you're building energized items anyways so you could start proc'ing it earlier.

If you're a real Sivir player you go Ghost Teleport for the ultimate zoom, but if you need the Flash and can't play without it then take Flash Ghost. You either want to pop ghost before an all-in, or when you need to run away from the big bad Warwick. For the Teleport, you can really use it anyhow, get to lane, gank another lane where you win 2v1 (not as an adc in 2020, but you're free to give it a shot), or in any other situation you see fit. Just don't tp into the middle of a teamfight, you will get deleted

Most items you build give you movement speed and crit chance along with attack speed, although you don't do nearly as much damage since your only ad item is going to be the Trinity Force, which isn't necessarily bad, but if you're going up against some adc which builds much more ad, you might feel weaker (depends on how much attack speed you and enemy has as well). Of course, you don't even need to go for Trinity Force, if you want a little bit more movement speed you can go for Turbo Chemtank, or if you need the magic resist you can build a Wit's End.

Read them, it's not hard. If you're too lazy:
Q is boomerang, throw forward, it come back. Deal damage
W is an AA cancel which deals more damage and travels to nearby enemies
E is a spell shield which blocks the next enemy spell (like Morgana e or Nocturne w)

You are playing farming simulator 2013. If your support is very aggressive, you can try and go for quick trades to proc phase rush and GTFO, but I recommend waiting until later in the game where you attack really fast, and you go very fast.

As adc, you're always in the back, so just do that. Hit the nearest enemy. Try and move with your support, or farm your lane until it pushes back. You can also travel with a tank or something to catch enemies off guard.

If the game goes too late, and the enemy has scaling champions, my best suggestion is to split push. Thanks to your mobility and general zoominess, it will be hard for enemies to catch up to you, and with your high attack speed you can easily proc phase rush. If split pushing isn't an option because the enemy Nasus decides not to leave his lane, you should group with your team and catch enemies out, then focus objectives, or you can try and siege their base if someone on their team is splitpushing

Just go fast man