As someone who plays a lot of MF, if you are struggling with CS max your E first, not only does it do relatively good harass early game but it clears waves quickly and allows you to pressure the lane a lot faster.
Since MF has low mobility outside of her passive, I suggest trying movespeed quints and taking swifties for a bit more maneuverability. Again a lot of people will argue but particularly in lower elo this will save your ass a lot.
You should basically focus plainly on farming during laning phase and ignore generally what the rest of the team may rage about. Ultimately if you win your lane, your doing your part for the team, if they lose then that's down to them.
One other thing, MF has a nice built in poke if you don't want to pressure the lane with her Q, basically you want to aim it at a minion and get the rebound onto a champion as it does more damage.
Later game if your laning phase went well you should generally be helpful to your team naturally, however do not forget that you have a complete lack of escapes so keep your distance and poke away, more so than other champs unless you are 100% sure you will finish someone off.
Ideally jungle fights against enemy teams are your specialty because of the cramps spaces your maxed E combined with R can pretty much penta teams. Just make sure you aren't wandering around solo and if you do get caught, hit your E to slow the enemy and run away before they get out of it (this is where movespeed comes in) usually by the time they get out of the E my passive kicks in and they have no chance at catching me.
