You get to do two things,

1. Swap a known spell for a new one. Basically forgetting the old. This is optional and done once per level.
2. Learn a new spell. This is mandatory and doesn't require forgetting one.

I’m also not a D&D player so the rules were new to me as well, but yes you need to forget a spell to learn a new one.

Then you can’t use it but next time you level up you can re learn it (forgetting another one) or I believe also learning it as the new spell.

When you level up, you gain new spells and have the option of switching out an old spell. Some level 1 spells like sleep are great at low levels but become obsolete later on, so you may consider a different level 1 spell with more utility later in the game.