In the early parts of the game a caster can't rely on magic alone - it's the limited mana and the spell cooldowns that really limit you, so I found that using a bow was essential (even without adding anything more than the default 1 skill point). From levels 1-10 the bow was my primary weapon, with it being a backup weapon from levels 11-15. Only once I'd reached level 15 (on Sentinel Island) could I fight by magic alone.
One point in the healing spell is probably the most useful at lower levels - it's going to save you a lot of potions and time waiting around for helath regeneration. In fact a single point in healing is going to be enough for most of your game (you get the most results from the first point spent, after that, the bonus is much smaller per point).
I personally focused on Fireball, followed by Magic Missile. Even capped at 5, those spells were often one-hit kills for non boss monsters as long as I kept them at the maximum my level would allow. I'd start with a fireball at maximum range (to kill/weaken as many as possible) then fire magic missiles until I ran out of mana before switching to the bow.
I think fireball was prefrerable to magic blast because of its area/mulit target effect. Magic blast is a good backup for tough boss monsters when fireball is still cooling down, but it wasn't worth spending points on until level 15+ (when I had lots of mana and enough spare skill points to focus on a new high-damage/long-cooldown spell). Even then I only found myself using it occasionally - fireball and magic missile were always my primary attacks, all the way to level 40 (spam magic missiles while fireball is cooling down).
Magic missiles have a major advantage that +magic damage enchants affect each missile, giving you anywhere from 2* to 7* the damage bonus per cast (depending on the level of magic missile, and the resulting number of missiles you can cast). However, it's not until you get your battle tower that you can start playing around with serious +magic damage enchants, which is why I gave fireball priority at the lower levels.
I tried some summoning early on thanks to a magic item that gave me two summoning skill points, but found it pretty useless - maybe someone who focused on summoning buff skills would have a different opinion.
Destruction is useful for increasing damage spells, but less effective than spending the points on fireball/MM directly, so I didn't start putting points into it until level 10-15 when I started to have some spare skill points. It's hard to say if you should chose destruction or magic blast as your third damage skill branch, but I'd favour destruction - as I said earlier, I only ever used magic blast occasionally, and once I had two damage spells maxed out, destruction bonuses were more useful.