If you are going against someone like Teemo, the way I usually play against him is by either going in on him hard in the early levels, because he is really squishy and doesn't really have much damage early on; or, I usually farm under tower, and try to find a way to engage while undr towr so the tower can help me, or the jungler can because he is so far overextended.
Honestly, if you look at Teemo, he is really only dangerous when he gets a few points into his E, and when he has some points in his Q. So around level 5, 6, that is when he can do some damage to you if he is any good. Prior to that though, he shouldn't be much of a threat. I usually have to play passively against Teemo early on, unless I can see him use his Q on a minion, and I can quickly get him low and make him engage early.
As for your second question, to prevent opponents from going in on you, don't use your abilities on CS. Unless you're going to be hitting the minion AND the opponent with your skill, it really isn't worth while both of you are in lane. A smart player will not engage on you, when both of you are full health, both of you have all abilities up, and all summoner spells up. Why? Because no one has an advantage unless the person you are facing knows you have a weak early game, and in that case you should be playing passively. Smart players only engage when they see mistakes being made, when their jungler is nearby, or when they know you're at a disadvantage through either a power spike in level, build, cooldowns, or what not. Early game laning phase really is about who makes more mistakes, otherwise it's just a game of CS.
If Rengar would not have used any of his abilities on minions, I would never have engaged, I would simply have farmed, as he was doing and wait till I either have a build advantage, a jungler gank for me, or a level advantage. A lot of the times it comes down to who you are playing and your opponent is playing, because champions that have strong early games will be aggressive against those with weak early games. But if we're thinking along the lines of two players playing champions that have the same early game, then it really comes down to who makes a mistake first or who gets a gank first. It could probably even come down to, who has a larger minion wave to do that extra damage. But quite frankly, I have never been in a lane matchup where both I and the opponent have played perfect lane phases, mistakes are always made, and the deciding factor is who capitalizes on the mistakes made.
