1) "When should i gank?"
that depends on a carry and on enemy. when playing ashe i gank every time i see that enemy has overpushed or is staying midlane with low hp.

main idea is that you shouldnt be seen going for a gank. push your lane, wait for creeps to move and run. it'll be some time before your enemy understand that u r mia, not just standing in fog of war.

press bluepill button when ur creeps move away so you ll bcome covered with for of war for the enemy. he might be just too late to call mia

go invis in line of sight of an enemy, he'll think u r going to get him and will be cocentrating on not dying himself to call mia

call for babysit if you r up against some strong pusher (like sivir)

don't doubt to much to leave lane for a gank. even if you don't kill enemy they'll be afraid to push too hard and that will make life easier for your allies