When I have to play a role that's not jungle or support

All you can do is practice. Play a custom game alone and see how much cs you get by 10 minutes or so. Repeat. Do this with a lane bot against you. Repeat.

My cs sucks too, I feel your pain

play nasus top alot

Oof, I feel ya mate. Not really helping that since I tend to fill, I'm increasingly often the ADC ^^;;

Farming at melee range is easier. But I have a hard time while farming with ranged champions.

Top is probably the easiest lane to learn to CS in. you don't have to worry quite as much about taking harass against another melee champion, and a top laner will usually have at least one AA reset built into their kit to help you if you're about to miss a creep. After you get the hang of that, transition to the bottom lane. Find an ADC who just feels good to play for you, for me it's Graves, and get a feel for their animations, and the travel time for their AA missiles. Get good about predicting when a minion is going to be low enough vs when you can fire a shot and try to get the two to sync up. It isn't easy at first, but it can be done. Also, get good at stutter stepping. If you don't know what that is, the simplest way to describe it is that you watch your champion while they're doing their AA animation and learn at what point they've sent off the attack. After that you can click elsewhere on the screen and cause them to start walking much quicker than they would have if you waited for their full animation to play. Clicking too early causes the entire animation to cancel, but clicking only after the shot has fired causes only the useless post-firing animation to be canceled, so you can keep on the move and keep repositioning.

What I found that helped was looking at my AD, and left clicking on the minion who was losing health the fastest. I'd wait until their life was about to match my AD and then right click, so by the time I went through the animation and fired the shot they were low enough to snag but not so low that my minions would take them. Or you can do it visually, by testing how much of a life bar you can chunk from a melee/caster minion when you first get into lane and waiting until their life bars reach that same amount to finish them off, but that involves a little more guesswork than the first idea.

Once you've gotten good at last hitting as an ADC, try it in the mid lane. Pick a midlaner who has REALLY smooth AA animations, someone like Ahri, Annie, Veigar, or Vel'Koz. Vel'Koz especially is someone I find it's easy to last hit with, as his AA animation is smooth and his missile is actually pretty much instantaneous since it's a bolt of void energy lightning stuff. His animation is so smooth, his attack is so fast, and even if I miss the farm at first I can toss out a W to make up for it to pick it up before I lose it, or liberally apply Ws to help myself catch back up if I'm falling behind in farm.

Consider what rune pages you're taking. For most top laners or ADCs, AD quints and armor pen marks is the standard set up. This gives you about 8 more AD and a solid chunk of armor pen right away, but there are some who might benefit more from taking flat AD in both quints and runes, or taking AD and AS quints/runes to increase the rate you're attacking instead of the damage of the attacks themselves, like Shyvana.

For top lane, I'd suggest trying Riven, Shyvana, and Wukong. They're all VERY forgiving on the last hitting and have AA resets to help you get the ones you're missing.

For bottom lane, I'd suggest Varus, Graves, and Ashe for practicing last hitting and stutter stepping.

For the midlane, I'd suggest Ahri, Vel'Koz, Annie, and Veigar. They all have different AA speeds, but in general the animations are smooth, and they'll give you practice with different AA missile speeds so you can get accustomed to whatever may come your way while being relatively forgiving in terms of when you fall behind, as they all have spells that are VERY easy to farm with.