I agree with some of the things jutschge said but not enough to not make my own answer.
Alright, most of your issues with laning will be solved by experience with your champion. As much as I dislike playing the same champions all the time I have started reducing the number of champions I play in ranked to around 10 as it hurts my early game when it comes to remembering how to last hit with that champion or how well they escape early game, etc.
When should I last hit?
When your not pushing a lane.
When should I harass my opponent?
Typically you wanna harass them by using an advantage you have over them to trade damage. This advantage could be a lot of things; range, heavy sustain, damage, defense, level, items. When you harass/trade with your opponent you should always know you are gonna come out and have done more to your opponent then they did to you. My favorite time to harass is if I have reached lvl 2 before my opponent and I will engage on them, use combo and get out. This is best done with champions who have gap closers, lifesteal/spellvamp, or cc. Another is when I have a massive minion wave on my side and as the last enemy minion dies I will cc my opponent allowing minions to hit pretty heavily before they can get away.
All easier said then done, as you don't wanna harass while they have a minion advantage but you also don't wanna not harass at all as they will slowly cut you down or quickly even. I typically only harass if I notice they are trying to harass me or I know without a doubt I will get them and lose very little in the process. I find that wasting everything I have for one kill is silly and is what costs most people their lanes. As more then likely the enemy will take advantage of you being without your summoners. As a jungler it is shocking the number of times I have come in and gotten an easy kill on a summoner who is a tick from health after dropping everything for fb. As now I have an advantage over the entire enemy team early game and can just acquire kill after kill.
When should you let your lane get pushed?
If your losing trades with your opponent, can't cs because of range, or wanna setup a gank.
When should you push your lane?
There are really only a few reasons to push your lane;
-first is your opponent can't cs under a tower and doing so gives you a demanding lead.
-second is you would like to base.
-third is you have a significant range advantage and can poke your opponent down even harder as they attempt to cs.
-fourth is you wish to roam or take advantage of your laning opponent roaming.
-last is to take a tower.
This one is easy for mid/top as once you have taken your tower you win. Although for bot you typically don't wanna take your tower as soon as possible if you are winning the lane. Being that ADCs need the most gold out of any other role they wanna sit and CS or feed as much as possible. Judging when your ready to take it is usually personal preference (typically I am comfortable taking a tower around 100 CS), just make sure you communicate if you dont wanna take the tower.
These are pretty strict as trying to set up for a kill or gank under the tower by pushing is a dangerous game, and just not worth the risk unless you got your whole team on standby. If your in a kill lane and your blitz/thresh can't land a hook in other parts of the lane they aren't going to any better under the enemy tower. Also pushing your minions to grab an early dragon isn't going to do anything unless your opponent has a dead/missing jungler and/or ADC. As no good team will just let you have a free dragon for a minion wave, especially if they have good vision.
Now some champions excel at pushing and thrive of off constantly pushing lane. Heimer and Singed are good examples. Singed is so good at it a form of pushing called proxy pushing, where you go past the first tower(while its still up) and grab minion waves. This is typically very effective at drawing the junglers and even the mids attention. And if done properly (heavy warding, jungler support, low enemy cc, lots of speed.) can shut down over half their team as they chase you around.
When should you freeze your lane?
Easy enough to decide here. Your enemy jungler is camping you pretty hard, your opponent has just pushed to tower then based. This will allow you to setup a 2 man gank as more then likely they wont have their lane warded. Most importantly is this is the best way to cs, and also the hardest thing to do when it comes to csing. Knowing how much or how little to hit creep so as to not push or not be pushed isn't very easy.
Hope I have covered everything, but if you want better answers you would be better off asking multiple and smaller seperate questions.