I've used many different methods for determining what spells magic-users (and sometimes clerics) have scribed in their first spell book when they enter the game. Sometimes it's all first level spells, sometimes it's just one, or one plus their INT modifier. Sometimes it's according to the (surprisingly restrictive?) table on page 8 of the Greyhawk supplement. Sometimes the spells themselves are picked by the player, or randomly determined, or sometimes a mix of both.
In your campaigns, what do 1st level magic-users get in their first spellbook? Please let us know!