With all types of spells, the number of hits you can get is capped at the Force of the spell that you cast. So with Heal for example, it doesn't matter if you score five successes on your Spell Casting + Magic roll if you cast it at Force 2. You'd have effectively scored 2 hits. The force that you cast a spell at is, in some way, how hard you are exerting yourself. Aptitude and skill can only do so much if you're not really trying.
Hope this helps. There are a number of popular misconceptions of magic. Somebody really ought to make a FAQ of them. The other most common one is not realising that drain of any type can't be healed by magic. If you search through the SR4 forum for "magic" and "overpowered", you should find quite a few interesting threads that cover a lot of this stuff.
Welcome to the game and welcome to Dumpshock! Keep asking questions and don't mind the grumpy types.