How do you react to getting countered ?

IMO, getting counter-picked is a lot more dependant on who's playing the counter than the fact that it's a counterpick. If the player is good, clearly it might be a rough time. However, if you're really good with your champ, you can win the lane easily. In a nutshell, skill > counters. That being said haha, some champs are just tough matchups regardless. For example, one of my favorite midlaners is Kat, but a mediocre Malzahar always manages to give me a hard time. Same with Zed vs Kayle.

You play smart, and wait for opportunities to get CS with little to no opposition. And you wait for openings to attack if you know you can get in damage without taking much. If you can't get openings to do damage, look to help your other lanes unless you're bot lane. Then just farm what you can.

Depends on how they are playing. I will be passive at first, poke them here and there to get a feel for how they are. If they aren't that good, I will get aggressive, if they know what they are doing, just stay passive til your jungler comes. Don't allow yourself to die and for gods sake don't repeatedly die to them. I see this far too often and its sad. Someone will die to their enemy laner, then try to all in them again(Even though they have Tiamat/BFsword/NLrod already) another 3 times and wonder why they keep losing.

Watch their play style. Most counters in League are soft or skill matchups, and especially in lower elos, people like to counter pick with champions they don't know how to use very well. There's a joke among people who tend to main only 1 or 2 champions that after a while, you pick up enough skill so that the enemy counter picking you goes to your advantage since you know the matchup but they probably don't.

Anywho, the answer is it depends on the matchup. Copout, I know, but think of it like this. In the simplest terms, you need to find your champions tools to deliver consistent DPS, frontload a lot of burst, or create a lot of effective health out of build paths and abilities. Then, you need to find out what the enemy champion is strong at. If you're facing against an AD Gnar top lane, a lot of his presence in mini gnar form is DPS through poking and his passive. In order to shut him down, the best trick is to frontload a lot of damage onto Gnar at once whenever possible. If you are familiar with his hop's long cooldown, you can time your engages to force him to activate the ability preemptively, back off for a second, then reengage the moment he gets too close again, this time without the ability to jump away. Give or take a Garen Silence or Xin Zhao Knock Up, and even a completely one sided melee vs Gnar match up can easily be turned around.

Simplified rule: DPS beats Tanks Tanks beat Burst Burst beats DPS

Basically.