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If an ability would produce one or more mana of an undefined type, it produces no mana instead.

Some abilities produce mana based on the type of mana another permanent or permanents “could produce.” The type of mana a permanent could produce at any time includes any type of mana that an ability of that permanent would produce if the ability were to resolve at that time, taking into account any applicable replacement effects in any possible order. Ignore whether any costs of the ability could or could not be paid. If that permanent wouldn’t produce any mana under these conditions, or no type of mana can be defined this way, there’s no type of mana it could produce.

To “tap [a permanent] for mana” is to activate a mana ability of that permanent that includes the {T} symbol in its activation cost. See rule 605, “Mana Abilities.”

An ability that triggers whenever a permanent “is tapped for mana” or is tapped for mana of a specified type triggers whenever such a mana ability resolves and produces mana or the specified type of mana.

A replacement effect that applies if a permanent “is tapped for mana” or tapped for mana of a specific type and/or amount modifies the mana production event while such an ability is resolving and producing mana or the specified type and/or amount of mana.

If an object’s enters-the-battlefield triggered ability or replacement effect refers to X, and the spell that became that object as it resolved had a value of X chosen for any of its costs, the value of X for that ability is the same as the value of X for that spell, although the value of X for that permanent is 0. This is an exception to rule 107.3i.

The tap symbol is {T}. The tap symbol in an activation cost means “Tap this permanent.” A permanent that’s already tapped can’t be tapped again to pay the cost. A creature’s activated ability with the tap 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. See rule 302.6.

The untap symbol is {Q}. The untap symbol in an activation cost means “Untap this permanent.” A permanent that’s already untapped can’t be untapped again to pay the cost. A creature’s activated ability with 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. See rule 302.6.

Any "enters" abilities of the copied creature will trigger when Altered Ego enters the battlefield. Any "as [this creature] enters" or "[this creature] enters with" abilities of the chosen creature will also work.

If the chosen creature has {X} in its mana cost, that X is considered to be 0. The value of X in Altered Ego's last ability will be whatever value was chosen for X while casting Altered Ego.

X can be 0. Altered Ego won't enter with any additional +1/+1 counters, and it will just be a copy of the chosen creature.

You can choose not to copy anything. In that case, Altered Ego enters as a 0/0 creature and is probably put into the graveyard immediately. It won't have +1/+1 counters placed on it by its ability.

If Altered Ego somehow enters at the same time as another creature, Altered Ego can't become a copy of that creature. You may choose only a creature that's already on the battlefield.

If the chosen creature is a token, Altered Ego copies the original characteristics of that token as stated by the effect that created that token. Altered Ego isn't a token.

Altered Ego copies exactly what was printed on the original creature (unless that creature is copying something else or is a token; see below). It doesn't copy whether that creature is tapped or untapped, whether it has any counters on it or any Auras and Equipment attached to it, or any non-copy effects that have changed its power, toughness, types, color, or so on.

If you copy a double-faced creature, Altered Ego will be a copy of the face that's up when Altered Ego enters. Because Altered Ego is not a double-faced card, it won't be able to transform. If an effect instructs you to return it to the battlefield transformed when it leaves the battlefield, it won't return and will remain in its new zone.

If the chosen creature is copying something else (for example, if the chosen creature is another Altered Ego), then your Altered Ego enters as whatever the chosen creature copied.