When should I play my land for my turn?
You can play a land for the turn during either main phase, before or after casting spells. There's no hard and fast rule about what time is best to play your land. If someone told you there was, they probably don't have a thorough understanding of the game. But if someone told you that in a specific situation you should've played differently, they might've had a point.

To give an idea, here are some of the things that might come up. These aren't meant to be exhaustive lists, just to demonstrate that it's not a black and white issue.

Reasons you might play a land before casting a spell or going through combat:


You need the mana immediately (you're about to spend all you can produce), or you're planning out actions in which you'll eventually want it all available and you want to make sure you count everything correctly.
The land has some effect that you want before playing spells (e.g. the scry from Temple of Silence).
You might need the mana to pay to keep a spell from being countered (if your opponent might have something like Mana Leak).
You have instants you could cast or abilities you could activate, and want to make sure you could do so if something happens in response to the other spells you're playing, or during combat. (Or you want your opponent to think this is the case.)
You want your opponent to think there's a lower chance you have other spells in hand.


Reasons you might wait to play a land until after casting spells or after combat:


You're going to draw cards and the land you'll choose to play depends on what you draw. (In some cases, this could even include choosing not to play a land at all.)
You want to find out what happens when you cast a spell or during combat before you decide what land to play.
You want to cast spells or go through combat with your opponent thinking there's a higher chance you have additional spells in hand, or a lower chance that you have a land (and thus that you have less mana available for spells/abilities).


Sometimes you might even consider not playing a land at all for the turn. Usually you don't want to do that since you might need the mana for a bigger spell or multiple spells later. But if that's not an issue, some of the same ideas as above come into play:


You might want your opponent to think you have more spells or fewer lands left.
You want to wait til later to get a non mana effect from the land.
You might have to discard a card later and would prefer it to be the land.


So, yes, there are definitely reasons it might be much better to play a land at one time than at another time - but it's not "always before" or "always after".