[Ardenya]

Once you hit 30 you can specialize in any of the casting schools. Whatever school you raise above 50 first will be your spec and will continue to go up till you hit the cap, whereas all others will max out at 50. As a benefit of specialization all spells associated with that school will have a chance to use less mana and fizzle less.

As a rule of thumb, most druids used to (and probably still are) specialize in Alteration since most our spells use it, especially those which use most mana eg. heals and ports but also others regularly used spells like snare and root. Thus, if you plan to duo/group a lot, Alteration would be a logical choice. On the other hand, if you plan to solo most of the time, stick with Conjuration since your most mana efficient damage spells, your magic based DoTs, are Conjuration-based.

Whatever school you decide to specialize in, to get it over 50, simply load up your lowest spell in this school and cast it over and over again until you hit 51.