First get a baby channeller from another dynasty. Second... Eat it.
Okay seriously, you need to do magic research. Channeling gives you +1 to essence, shard harvesting lets you build shrines on magic shards, each one increases your essence by one. There are also several other buildings in the magic research tree you can unlock to increase... Finally there is a recurring research item in the magic research tree that gives you a bonus every time you select it.
In addition to this you can increase it by 3 every time you level up (Stat might be titled wisdom, pretty sure it's essence once in game though).