Alignment is two axes crossed: how your character regards other people — good, evil, or neither — and how they regard order, from lawful through neutral to chaotic. Nine combinations, and your character is one of them, or deliberately none.
Alignment gates nothing. No class is closed to you because of it, no spell checks it, and no feature turns on or off according to what you wrote in that box. It is a two-word summary of how your character tends to behave, written down so you and the DM have a shared expectation — and if play contradicts it, the honest fix is to change the box, not the character.
Treat it as the least binding thing on the sheet. A lawful character can lie; a good one can be cruel when pushed. Alignment is where the needle usually sits, not a rule about where it may point.
The rest of this step has no mechanics attached, which is exactly why it is worth spending time on. A character becomes playable when you can answer:
None of this is required, and none of it is fixed once written. The first two sessions will tell you more about who this person is than any amount of preparation, and the sheet is allowed to change to match.