Above all, you must have a desire to lead and impress people in a peaceful way.

Now to brass tacks:

The main business of a caller is to call, i.e. to give commands in a rhythmic way with the music. For this, you may learn some singing calls, or you may learn to call contras. How? This is described here

In 1970 ff, Sets in Order came forward with some books "The Caller-Teacher Manual for ...", and ECTA means "European Caller-Teacher Association". Teaching is the next important part of our business. For this, you must know and understand the definitions, but not use them in your teaching. Use your own words, show, let the people practice ... What do other callers do? Emulate or avoid.
I myself find Contra Dance an important Teaching Tool (Handout of my seminar at the ECTA-Convention 2005)

Then find flowing choreo. Here again, you can find a lot in contra dancing.

Resolving is my weak spot; therefore I consider it the least important item, though it is the prominent part in many caller schools. [Sour grapes ;-)]

Read also:
Caller Seminar by Don Armstrong
The Direction of our Caller's Training, by Dick Leger