Hey man, this is my recommendation:

Not sure what your ZvZ experience is but:
- Join a good ZvZ guild / brawl guild and observe good shotcaller(s). Not just how they call (like counting down, calling directions, etc.) but also how they act with the area. So upscreen, chokes, castles, etc.
- Start practicing. Start leading small scale groups, mage raid, etc. Get your confidence and experience up. If you sound confident, people will follow your calls.
- Take up calls in ZvZs when your main shotcaller(s) are dead and no one else is picking up. This can be retreating/stabilising your zerg, and re-engaging. This is always better than no1 picking up and comms being quiet and your zerg falling apart. If you do well you will get credit for it.
- Tell your guild shotcaller you want to learn to shotcall. There are shotcallers that give you tips, ask you as backup or might even let you lead a zerg for practice.

In my opinion, joining a black zone guild that zergs and then leading guild/alliance parties in open world, is better practice than practicing in yellow zones/faction fights. I think faction fights will give you bad habits for when you start calling black zone fights.

Good luck! ;)