3-8 - Randomly determine 1 1st level spell.
9-12 - Pick 1 1st level spell.
13-15 - Read Magic and pick 1 1st level spell.
16-17 - Read Magic, pick 1 1st level spell, and randomly determine 1 1st level spell.
18 - Read Magic and pick 2 1st level spells.
Inspired by my recent academic adventures, I imagine the above as this:
Int 3-8 - You're inept; although you somehow stumbled into an apprenticeship/wizard school, you were a failure and only remember how to do one thing, and it's probably not want you wanted to learn.
Int 9-12 - Although you got into an apprenticeship/wizard school you're pretty much a hack and managed to only learn how to do one thing.
Int 13-15 - You were an adequate student, you managed to learn a spell and the skills necessary to learn more on your own.
Int 16-17 - As above, but you managed to also retain some scraps of random information.
Int 18 - You were a prodigy and graduated with two spells of your choosing, what a champ!
One can easily play around with this depending on the DMs preferences regarding starting # of spells, randomization, etc. Maybe you need a 13 Int to avoid random starting spells? Maybe an 18 Int PC gets to pick 3 spells. I just like having a handy, arbitrary system for determining starting M-U spells that allows for some reward for being a brainiac.
I always liked the AD&D starting spell determination in principle, but in execution is a mess however much I love the cheap laffs at the expense of 1st level M-U's that have Friends, Mending and Ventriloquism in their spellbooks ("Where did you learn magic, working at a cricus? Bwahahahaha!").
As an aside, random spell determination works a lot better for the PCs with the OD&D/B/XD&D spell lists which aren't cluttered up with the hedge mage spells of dubious general utility. Heartbreak is your Magic-User reaching 7th level and getting Shatter in their spellbook. Not that I'm entirely against the D&D rules repeatedly slapping your PC in the face.