Clerics know all the spells available in the Cleric spell list. They can prepare a number of these spells equal to their Wisdom modifier + their Cleric level, as long as the spell is at most the same level as the spells slots the Cleric has access to. When using the spells they 'burn up' one of their available spell slots, which at level 1 would only be 2 1st level slots.

This means that a wise 1st level cleric can have several different spells all prepared at the same time, but will only get to use two of them before having to take a long rest.

This is different from how Sorcerers operate, since they both can have a number of spells learnt but don't exhaust the spell when it's cast, so they can keep firing off the same spell as long as they have the slots but can't change what spells they know until they level up.

And then the Warlocks just read the pretty words from their book of stolen magics, and so can fire off spells they've written down without needing to prepare them in any way.

[i]Cleric:[/i] [color=lightgray]Knows all spells, must prepare them while resting before using, limited in uses by spell slots.[/color]
[i]Wizard:[/i] [color=lightgray]Must learn spells (copy into spellbook when encountered or learnt when levelling up), must prepare them while resting before using, limited in use by spell slots.[/color]
[i]Sorcerer:[/i] [color=lightgray]Must learn spells (memorise when levelling up), may use any learnt spell without preparation, limited in use by spell slots (but may generate extra spell slots without resting.)[/color]
[i]Warlock:[/i] [color=lightgray]Must learn spells (learnt when levelling up), may use without preparation, limited by use in spell slots.[/color]