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