The first thing to say is that this game is all about the Wizard. There are different types and they do different things so it is really important to pick a wizard type that suits your style of play. This is key to enjoying each game.
Casting - its all about casting. You will need to be casting spells every single turn. This is true for both the wizard and the apprentice. This will give you bonus XP after each game. It is how you build your wizard and warband.
This means that you will need to choose a low cast value spell, from your wizard school, that can be cast on yourself or the environment. You can't always see the opposition or indeed members of your own team so this is important. This will be your 'go to' spell. It will have a big impact on how each game feels. Over time this can change but for the first few games it will be very influential.
Spells having a low casting value is important especially for your Apprentice who added +2 to the dice roll. If you are casting Aligned spells there is a further +2 penalty so review your Wizard type carefully. Low value environment spells are the ones you are most likely to be casting.
The lowest cast value is 8 so it would be really helpful to have at least one spell in your armoury with this value. Not all schools have that as an option though so then you are looking at 10 as the lowest cast value.
I choose to play an Elementalist. (I have no spell option with a cast value of 8.) I did this because I like the idea of casting balls of fire. I have managed to do this only occasionally during games. My most commonly used spell is Wall. This is because I can cast it on any turn. I don't need to see anyone else and it has a casting value of 10. Another option for the Elementalist is Shield, which has also a 10 cast value, as this is cast on self only.
My Elementalist has become all about putting up walls. I did not see this coming so it worth reviewing the options before making your starting choice. I have done some of the work below: -
Chromancer has a few low value spells. Fleet Feet at 10 is the spell most likely to be cast a lot. The other potential option is Fast Act which has a low cast value of 8 but you need to have LOS.
Signalist has Explosive Rune. This has a cast value of 10 and is cast on the environment. It is like planting a grenade that can be set off by the opposition. A good option but not a starting 8.
Soothsayer has Awareness. This has a cast value of 8 and cast on self only. It adds +1 to Initiative rolls for the Warband. It doesn't stack but could be cast multiple times to have a successful cast bonus. A bit dull though for your 'go to' option.
Summoner has Leap. Cast value of 8. Move a figure (including self) 10". Handy to have. I really like this option and it is great for moving models encumbered with Treasure which slows movement to half.
Thaumaturge has Heal. Again this has a low cast value of 8 and could be easily cast all the time even on self. Again its a bit dull as a 'go to' option.
Then we have Witch. There is a good low value spell here which is Fog with an 8 cast value.
The other thing to consider is that you are likely to spend your first few improvement points on your 'go to' spell. So it becomes more reliable. Of course because its more reliable you will use it more etc etc. So make sure you like it. I am kind of locked into Wall now and it can be frustrating in games.
Hope this helps you make your decisions.
Most of all just pick the school that suits you and have some fun.