Both are things that will come naturally to you the more you play. Positioning is just keeping in mind places you can go that you are least likely to die, and you'll start doing it with very little thought after getting more experience with the game. It's mostly thinking about, say, which way is the safest path? are teammates in this direction? do we/they have wards in this area? and consider the enemies that are near you. If a Gnar or a Vayne or someone who can pin you to walls are around, try not to get too close to walls. Do they have a Sejuani or and Orianna or something similar? Don't hug your teammates. And when you're grouped, always try to have your tanky teammates between you and their DPS.
Last hitting will also get easier with practice, and with full rune and mastery pages. If you want to push a wave with a full 6 (or 7 w/ cannon) wave on both sides hitting eachother, just hit the ranged minions, and go for last hits on melees. If you're constantly AA'ing the same creeps as your minions are, it's much harder to get the last hit. If you have a habit of constantly autoing the waves, try to break that habit quickly, it really does make CSing harder. You want to be in control of the wave. Push only when you want, and freeze when you want. Conservative use of spells on champs that have resource pools to last hit is fine, and if you're playing someone like Lucian or Syndra, you can use a skill to last hit and harass the enemy laner at the same time.