a wizard has a good start with blink and mephitic cloud, but will have problems with anything poison resistant like an early ice beast. if you worship vehumet you'll be guaranteed higher level conjurations or summonings (whichever is highest first) but it'll take a while.
air elementalists have a well-rounded book with repel missiles and swiftness, but lack firepower until you get lightning bolt, which isn't all that good anyway, but it's a good background.
conjurers and ice elementalists have very strong starting books. conjurers have early access to mephitic cloud, and ice elementalists have throw icicle which is a very strong level 4 conjuration (even after the nerf it's only partly resistible), and both have freezing cloud which is very strong and crazy efficient.
earth elementalists are a bit weird but very strong. sandblast with stones as a reagent (wield, then cast it) and conjure arrow will carry you for a while, and lee's rapid deconstruction is fun. later on there's iron shot and lehudib's crystal spear if you're also training conjurations.
necromancers are fun but if you want to try that out better start as something else and then worship kiku. he's special in that he gifts books very early (at one, three and four stars of piety if i recall correctly). agony trivialises pretty much anything other than uniques and packs of death yaks in the lair.
summoners are crazy strong but their starting book has a ceiling and the interface is annoying, so noone plays them (i do, but it takes patience). transmuters are also strong and have a good early book, but i'm not sure that's what you want. they pair up very nicely with kiku too.
there's plenty to try out, so i'd avoid fire elementalists (no utilities these days and other schools are just as good as killing things dead), warpers, venom mages (same problems as wizards, even though a few venom bolts will kill an ice beast), enchanters (they are strong but nearly all their spells are resistible, which is frustrating to some), and whatever else i've forgotten. ah, skalds are also pretty damn good, but they have a very rough start training gazillion magic schools and are a hybrid magic/melee class anyway.
race is much more important than class, but not so much for a beginner, so just look at the aptitudes ('%' in the character selection screen) and choose something that's not terrible. deep elves and kenku are very fragile, naga are slow, the undead are not very beginner-friendly, etc. some people like spriggans, others don't, but in any case they just play a different game than any other race. high elves are nearly always good, but level up slowly. humans are not a bad choice, as they are just about impossible to mess up.
also: if you use magic *at all*, level up intelligence whenever you can (put some points in strength for carrying capacity if it's driving you nuts, but that's it).