You may have this creature enter as a copy of any creature on the battlefield.
Clone's ability doesn't target the chosen creature.
If the copied creature has in its mana cost, X is considered to be 0.
If Clone somehow enters at the same time as another creature, Clone can't become a copy of that creature. You may choose only a creature that's already on the battlefield.
Any enters abilities of the copied creature will trigger when Clone enters. Any "as [this creature] enters" or "[this creature] enters with" abilities of the chosen creature will also work.
If the copied creature is copying something else (for example, if the copied creature is an Evil Twin), then Clone enters as whatever that creature copied.
You can choose not to copy anything. In that case, Clone enters as a 0/0 Shapeshifter creature, and is probably put into the graveyard immediately.
Clone copies exactly what was printed on the original creature and nothing else (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 Auras and Equipment attached to it, or any non-copy effects that have changed its power, toughness, types, color, or so on.
If the copied creature is a token, Clone copies the original characteristics of that token as stated by the effect that created the token.
in its mana cost, X is considered to be 0.