Kiwi - League of Legends

This is a common topic, normally in solo queue normal games, nobody really cares about warding, they might get some wards after they first b to prevent ganks, but that's pretty much it.
Ranked solo queue is a lot better, but the best is archieved when all team cooperates on warding, and this is mostly on arranged teams.
Surprisingly most people don't know where to ward, and most guides just throw an image with 100 possible ward locations, this is awesome but you have to consider the game state too, normally you will have different wards on early, mid and late game, and position change a little sometimes depending if you are blue or purple team.

Warding to prevent ganks
Early game wards are for preventing ganks, and basically you need full river sight. Spots for wards change a bit depending if you are blue or purple.
These are some nice wards positions for blue and purple team.

Blue Team Early Game Wards

Purple Team Early Game Wards

By warding on those locations you will have full sight of the river, and any jungler ganks. There are some variations too, for example, when playing on purple team, you can ward the bush at the bottom right of the dragon, instead of the river, but it's not much difference. Try not to put the ward in the river bush because reaction time is significantly lower if you put it there. Notice the dragon is not warded, that's because you don't really need to see it, you know it's there, and you can see if they try to take it. After early it's nice to ward it though.

Some Late Game Wards
Mid-Late game wards are a bit different, you must ward baron and dragon, and depending on how aggresive your team plays or how much your team push, you will want to ward their jungle too.

Purple Team Possible Wards

In the image above you can see the possible wards for a purple team late game. Warding the enemy jungle allows you to fight them on the jungle, which is a nice advantage sometimes as you normally can pick up people one by one. You should also consider warding your own jungle if they are invading.

Who should get wards
As a carry, never buy more than one ward at the same time, it will slow down your build. Of course there are some exceptions but normally one per shop is ok.
As a jungler you should get some wards for river, and if needed, oracles.
As a support you need to get a lot of wards, if you don't have oracles and the enemy team also doesn't, consider getting a pink for dragon/baron, those places are always warded. Try to get between 2 and 3 wards everytime you shop.
All in all, the role who must get the most wards is support, followed by jungler and tanks, finally carries.

Ward when you overextend
To overextend safely, once the first lane tower is down, you should ward if you plan on pushing that lane alone, up to the second tower, that way you will be safe.
For example, If you are purple, and you push top, you'll ward at their blue.

Tips

• If you put a ward inside baron the enemy team wont be able to kill it, even with oracles, it's a well known bug right now, which Riot surprisignly still hasn't fixed.
• If you know the enemy team is warding a lot and you are a support, consider getting oracles.
• You can check out TheOddOne's jungle video, part 2, where he talks a bit about warding to prevent jungle ganks.