I would try to play a few custom games with no one else and just practice last hitting to get the timing of the auto attack animation down. Once you do this try to figure out how much dmg you need to be able to last hit under your turret with the tower doing 2 hits to fighter minions and 1 hit to caster minions. When I play Ezreal I tend to buy 2 long swords on my first back because that puts me just over the dmg I need to be able to do this. CS is the most important thing in this game. If you cant get the money to buy items you will be worthless completing 1 item after 25 mins while your enemy is sitting there with 2 and a half.
With Sivir you should also figure out if you want to push the lane or not. If not dont use your spells on minions. You can just sit there last hitting and if the enemy tries to push the lane to your turret you can lower the minions with AAs then use your Q to take down the entire wave so it doesn't get to your turret.