If the chosen permanent is a token, Malleable Impostor copies the original characteristics of that token as stated by the effect that put the token onto the battlefield, with the listed exceptions. Malleable Impostor does not become a token.

If the chosen permanent is copying something else (for example, if the chosen permanent is a Clone), then your Malleable Impostor enters the battlefield as whatever the chosen permanent copied, with the stated exceptions.

If the chosen permanent has in its mana cost, X is 0.

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

You can choose not to copy anything. In that case, Malleable Impostor simply enters the battlefield as a 0/0 creature and is probably put into your graveyard immediately, unless something else is increasing its toughness to keep it alive.

Except for the listed exceptions, Malleable Impostor copies exactly what was printed on the original permanent and nothing more (unless that permanent is itself copying something or is a token; see below). It doesn't copy whether that permanent is tapped or untapped, whether it has any counters on it or Auras attached to it, or any noncopy effects that have changed its power, toughness, types, color, and so on.

Any enters-the-battlefield abilities of the copied permanent will trigger when Malleable Impostor enters the battlefield. Any "as [this permanent] enters the battlefield" or "[this permanent] enters the battlefield with" abilities of the chosen permanent will also work.