How do you determine the Spellcasting ability and the spell attack bonus? I've tried searching for it in the player's handbook and searched for answers here, but I haven't found anything.
Page 114 of the player's handbook under the Spellcasting Ability header lists the ability and relevant spell modifiers for wizards:
Intelligence is your spellcasting ability for your wizard spells...You use your intelligence whenever a spell refers to your spellcasting ability:
Spell save DC = 8 + your proficiency bonus + your Intelligence modifier
Spell attack modifier = your proficiency bonus + your Intelligence modifier
Note that the same rules can be found on page 32 of the basic rules