I mainly need help with which skills to take and what feats to take. This in only my second character and first Wizard build. Also the highest level I have gotten to so I don't know what is needed for higher level quests.

I am currently a lvl 15 Human Lawful Good Male Wizard (32-point build)

Stats without enhancement bonus
STR - 10
DEX - 10
CON - 14
INT - 21 +4 feat bonus
WIS - 10
CHA - 8


I have currently been investing the following skills:

Main Skills:
Balance
Concentration
Diplomacy
Heal
UMD

with a couple of points going to Haggle, Jump, Repair, Search & Tumble

I have the following Feats:
Empower Spell
Extend Spell
Heighten Spell
Greater Spell Penetration
Improved Mental Toughness
Insightful Reflexes
Toughness
Spell Focus: Necromancy

I know that I have taken some bad skills and feats along the way and I intend to use a heart of wood to correct these issues. I plan on focusing on fire and cold spells since I have heard those are the most effective agains most foes. I could also use some help with which enhancements that are "must have".

Also, does it matter at which level you use tomes at? I intend to get the +2 tome of supreme ability soon and I will hold off if it won't result in much.

Some help will be greatly appreciated and sorry for the long post. If any more info is needed to help let me know.