Spellcasting for sorcerer

So I’m playing a sorcerer and I realized I might have the spell slots understood wrong.

For simplicity’s sake, say I only have one level 1 spell slot. After a long rest, do I have to prepare one spell (say it’s magic missile) that I can use during the day (meaning throughout the day, I can use only magic missile), or can I just use absolutely any spell as long as it fits the criteria (i.e. it’s only one level 1 spell)?