hey, everyone here is learning. So, no worries. There is ALWAYS new things to be learned, ask away.
Yeast is an interesting "ingredient" since it is alive. Like Sean says, experience is definately the best teacher. Even then it can throw you for a loop even using the same exact type becasue since it is alive, it changes.
Most yeast producers make recommendations as to their yeast strains flavor and temperature profiles and "suggested" beer styles. There is also attenuaton and flocculation rates, i.e. how much sugars they will consume and how readily they sediment. Following recipes and brewing software will also give you a feel for what yeasts to use. Once you think you have it figured out... You experiement!
Personally, I do not think I will ever truly claim to "undertand" the various yeasts, it is a bit daunting, but you can definately learn to use them and use them well.
Are you using dry or liquid yeasts right now? Dry has less strains available, liquid... mind boggling!