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.
- This only affects creatures, not other permanent types.
- This does not tap the creature, it just prevents it from attacking or using it’s tap (or untap) abilities.
- Creatures with summoning sickness can still block (as long as they are untapped.
- Creatures with summoning sickness can still be tapped by other effects. For example, if you had a Merfolk creature enter your side of the battlefield this turn, you could still tap it to pay for one of Kumena, Tyrant of Orazca's abilities.