No, casting a cantrip replaces all of your attacks for the turn. Each turn (by default) you have one action to use. A few examples of actions are: Attack, Cast a Spell, Dodge, and Dash (see the Actions in Combat section of the PHB for more).

Do Cantrips count as prepared?

Cantrips don't need to be prepared, and they don't use up Spell Slots.

Are Cantrips actions or bonus actions?

Yes. If you have a cantrip which you can cast as a bonus action, you may cast it after attacking. Sorcerers can also spend 2 sorcery points to quicken a cantrip so that it may be cast as a bonus action (if they know the Quickened Spell metamagic).