A good rule of thumb is to always ward objectives (drag and herald), if you are playing top you can ward the T brush to avoid a jungle gank, or you can try to ward the enemy jungle to know if the jungle is pathing to top or adc.
In my opinion, the best places to ward are places where you think YOU will go if you were the enemy. Places like bushes and areas where you will not go are very popular.
for good wards you must know the enemy's gank possibilities, it's not the same an evelyn lvl 2 gank to one with jarvan/shaco lvl 2, even a zac, in short I recommend you to analyze the enemy composition and see how possible is a gank or a botlane roaming, some champs are more of powerfarm, if you are in group with the jg you can abuse a fiidlesticks, evelyn pre lvl6 as long as you have line pressure and do good tracking to the enemy jungler.
It also depends on the different stages in the game, how far into a lane you are pushed/pushing. If you are under your tower, you are less likely to need a river ward. Top vs bottom lane, playing top vs playing bottom can also impact where you place.
I agree with what has been previously said. Your vision should be defensively placed in the sense of watching for ganks. You do not necessarily require anything more than your trinket ward early on.
buying one pink could benefit you so much as they prob might not clear it all game but later elos you might wanna just double up just in case they get insta clear by enemy laner/jg
