I'm new to Magic: The Gathering, so I am not familiar with the rules yet.
The person who taught me says land cards do not enter tapped, unless the card says otherwise, meaning I can have land enter the battlefield, tap it and use it to summon a creature onto the battlefield.
According to others I played with, ALL CARDS enter tapped, even basic land, and says I have to wait until my next upkeep to untap, then re-tap the mana in order to use it for summoning a creature. Which is correct?
All permanents, including lands, enter the battlefield untapped unless an effect says otherwise.
This is stated in the rule book, in rule 110.5b:
Permanents enter the battlefield untapped, unflipped, face up, and phased in unless a spell or ability says otherwise.
As murgatroid99’s answer states, per rule 110.5b from the comprehensive rules, everything enters untapped unless something says otherwise (as well as unflipped, face up, and phased in).
However, it's possible that the statement that everything enters tapped may have been true at that particular point in that particular game. There are a couple of cards that actually have this effect (though none are currently standard-legal), such as Orb of Dreams, or a tapped Archelos, Lagoon Mystic. These further highlight though the fact that normally things enter untapped, because otherwise they would be pointless cards (and WotC generally does not print truly pointless cards).
It’s also possible that the players who told you everything enters tapped are confused about rule 302.6, commonly known as ‘Summoning sickness’:
A creature’s activated ability with the tap symbol or the untap symbol in its activation cost can’t be activated unless the creature has been under its controller’s control continuously since their most recent turn began. A creature can’t attack unless it has been under its controller’s control continuously since their most recent turn began. This rule is informally called the “summoning sickness” rule.
Notably:
