You cast the spell at its lowest level.
Scrolls are defined by the spell they contain. If they contain a 1st-level spell (like Magic Missile), then they are a Spell Scroll (level 1).
From the DMG (page 141) or online:
Some magic items allow the user to cast a spell from
the item. The spell is cast at the lowest possible spell
level, doesn't expend any of the user's spell slots, and
requires no components, unless the item's description
says otherwise. The spell uses its normal casting time,
range, and duration, and the user of the item must
concentrate if the spell requires concentration.
You can also find info on Scrolls on DMG (page 139), or more specifically on DMG (page 200). On-line resources are here:
If the spell is on your class’s spell list, you can read the scroll and cast its spell without providing any material Components. [...] The level of the spell on the scroll determines the spell’s saving throw DC and Attack bonus, as well as the scroll’s rarity, as shown in the Spell Scroll table.