If you're a
bard or
sorcerer, you can select any spell you know, provided you are capable of
casting spells of that level or higher.
To cast a spell, you must be able to speak (if the spell has a verbal component), gesture (if it has a somatic component), and manipulate the material components or focus (if any). Additionally, you must concentrate to cast a spell.
If a spell has multiple versions, you choose which version to use when you cast it. You don't have to prepare (or learn, in the case of a
bard or
sorcerer) a specific version of the spell.
Once you've cast a prepared spell, you can't cast it again until you prepare it again. (If you've prepared multiple copies of a single spell, you can cast each copy once.) If you're a
bard or
sorcerer, casting a spell counts against your daily limit for spells of that
spell level, but you can cast the same spell again if you haven't reached your limit.