As for sj at 20, your best choices are /blm and /sch. /rdm doesn't come into its own until post 75 really, but it wouldn't be bad to get it to 50 so you don't have to worry about it later.
And yeah, there are spells as a 99 whm I rarely use myself. All the cura spells are not used very much, for instance; I'd recommend though that you get your debuffs (for if you ever play another mage job so you can have them already), all elemental bar spells (Save your money and get the status ones when you got extra gil; they are not always that useful), all nas or status remover spells (this includes Sacrifice tho that don't come into its own before the higher levels btw. Esuna is also one you should get, it is a ga status remover for when you got it on you too), regens, haste, cures (this includes the curagas, don't worry about the cura series so much), protectra/shellras (Get the single target ones when you get extra money; they are not used as much but I find them handy for when Stupid Puller runs when you let'er rip) sneak/invis (Don't worry about Deodorize unless you get it as a drop, that I swear is more for the classic "YOU STINK!" joke; aka casting it on someone and saying they need a bath), the stat raisers (Most impt one is str imo, so get that first) Flash (I rarely use this now, rusty but it can help), Repose (good for oh shii situations, again not one used much but when you need it you REALLY need it), raises (I won't yell at any whm without Arise but get them up to R3, and save for Arise), teleports(Don't buy these. Work up your fame and quest them), blink/stoneskin (for when you're getting beaten up or have a chance of getting beaten up) and Auspice (I need to use this more, it really helps.)
Anything else is more for complete imo; get the missing ones when you got money to spare. I think that about covers it. ^^
Elizara, Mithran WHM of Quetzalcoatl
LS's: SpecialFriends, ShikigamiWeapon, Noble's, WeSayHurray, JingZen, Betrayed (Dynamis and Aby)
Still a MithraPride kitty at heart, tho that shell is gone..Also still CTY at heart forevah!
Midgard: NEVER FORGET.
Alla profile: http://ffxi.allakhazam.com/profile.xml?11530
Thinking about swapping from console to PC? Check here to do it right!