With a team that will work with you, the best warding strategy ends up looking something like this:
- Top/bottom lanes ward river bush (or nearby) and tri-bush
- Mid wards the bushes on either side of their lane or the bushes just beyond the mid bush that curves around into one side of the river.
- Jungler wards buff mobs and dragon/baron.
If you search, you'll find some in-depth maps that tell you about additional spots you can ward that will be helpful. In general, though, you should get used to at least warding the buff jungle mobs and the dragon/baron as the jungler. Here's a simple map (which I stole from Mobafire) that illustrates a simple warding strategy:
The areas the jungler would ward are the ones marked with red, orange, and yellow stars. An aggressive jungler will ward both yellow and orange so they can protect their buffs and either steal their opponents buffs or gank the opposing jungler when he goes for his own buffs.
As for types of wards, you'll default to the green (75 gold) wards. Purchasing the pink (125 gold) wards is typically reserved for warding baron and dragon since you'll likely be able to uncover a ward and kill it, thus removing map awareness from an enemy.
In addition, if you see an enemy place a ward in a place you'd usually place a ward and you're going to ward the area within 3 minutes (the duration of wards), do it with a pink ward instead of a green ward. You'll be able to kill theirs (which will get you 25 gold, thus making your end cost 100 gold) and you'll make things more difficult for your opponent.