Oracle Text

As Conspiracy enters the battlefield, choose a creature type.

Creatures you control are the chosen type. The same is true for creature spells you control and creature cards you own that aren't on the battlefield.

Card Rulings

10/4/2004 It does not replace any use of creature types in card text.
10/4/2004 This can grant a creature type to animated lands and artifacts that would otherwise have no creature type.
8/1/2005 “Legend” is no longer a creature type, and may not be chosen.
8/1/2005 If you choose Wall, then your creatures can still attack because creature types don’t confer abilities such as defender (any more than, say, choosing creature type Bird would confer flying).
9/20/2013 Affected spells, cards, and creatures lose all of their other creature types.
9/29/2017 Replacement effects that modify creatures of a certain type as they enter the battlefield will apply (or not apply) after you apply this effect. For example, if Warrior is the chosen creature type and you control Bramblewood Paragon, a Runeclaw Bear would enter the battlefield with an additional +1/+1 counter.