Is there a good map on common ward placements or something? As I ma starting out I mostly guessing where to put them best.
During lane phase, as a laner, your main priority is to make sure you don't get ganked, so you want to ward near your lane (bot blue side would ward enemy river bush, or just river if that's too dangerous, red side bot would normally try to ward tribush). As a mid laner you often ward one side and lane towards that side. A really valuable ward, if you can get it, is at enemy raptors, as that's a high traffic area where you can see where the enemy jungler goes after he clears it (same with wolves or enemy jungle generally, but they're harder to place and less high traffic). Generally there are more wards in the bot half of the map than the top since the support, who normally buys Sightstone, lanes there.
Once you get near the end of lane phase, roaming becomes more frequent and dragon sneaks become a real possibility. Therefore seeing as much of river as possible is helpful, especially around the dragon pit. If you've lost a side lane tower keeping a ward at the nearby entrance/exit to your jungle is important as it should help you keep control of your jungle. If you've lost mid lane tower you need to work hard to keep mid shoved out so you can contest the middle of the map (there's so much area the enemy can access that wards only help so much without minion presence).
If you're ahead, you're job becomes clearing vision. Supports and junglers will usually swap their yellow trinket for the red trinket, which at level 9 can be upgraded so that it rolls along with you for a few seconds and can be used to deward a big chunk of the map. The 75g "pink" wards have an unlimited duration and also reveal and disable any wards in their vicinity, so they are very powerful for controlling the map. But don't place them where they can't be defended (they are almost never placed outside bushes unless inside baron or dragon pit), as they are visible and easily removed by enemies.
Once you get beyond 25 minutes or so, lots of towers will be down and the big danger will be baron. You want to make sure there are wards around the baron pit so it can be snuck away by the enemy team. Pink wards in your jungle are valuable to so that traversing between lanes and camps is safe for your carries (e.g. pink the bush by blue buff). If you want to force baron (either you make the enemy team come and you engage on them, or you take it and push) you should always have a pink ward to place inside the pit so that it's very hard for the enemy jungler to steal it, and some wards inside the enemy jungle to see them coming. Ideally you will have swept the river and know they are walking in blind so you can easily turn on to them with a big advantage.
As you play more you start to get used to where and when vision is useful and where it is just a waste of time. If you're playing against assassins you'll want more wards in "niche" spots to prevent them from sneaking into position to pick off your carries. If you're against a relentless splitpusher you'll want more wards in your own jungle so you can see where he goes when he's not in lane and whether you can collapse him (or he can collapse your team). If you're winning hard you want lots of dewarding resources to lock the enemy team out of their jungler and starve them more than you want extra wards since the enemy will be visible most of the time anyway. If you're losing hard you want to ward defensively just to avoid being killed as you farm and can see where enemies are moving so as to play around them.
Vision is also sadly ineffective at low ELO due to low map awareness so you need to ping a lot to get your team to make use of it (i.e. you see the enemy jungle walking into river from his raptors while your mid is pushed out; immediately ping him to back off as he's vulnerable to a gank, he probably wasn't watching the mini-map).
This guide has some good ward spots with explanations:
https://www.youtube.com/watch?v=fsjPxpdu5eM
So does this guy here (bit more depth, focussed on early game):
https://www.youtube.com/watch?v=AbUqScdHbAw