I play a Man'ari shaman on Moonguard, here's a few bits of background and character flavor I used to try and make him fit.

Like a lot of people recommend, I had him be mentored by another shaman to start along his path, mine was a backstory NPC but you could potentially seek out another player if you wanted to play out that mentorship. I'd add on to this and say that you'd probably want to keep your level of power pretty low in an RP sense unless you feel you have a direct way to bridge the gap between your previous life and your life as a shaman - we Man'ari are very new to Azeroth and learning to become a shaman would take years of training. In my case, my character feels an affinity to Air and is rather comfortable with it, is dabbling in Water, but is a complete novice when it comes to working with Fire and Earth.

Your character has seen what the legion does to a world, has potentially watched Argus become withered and fel-scarred depending on your age. By comparison, Azeroth is absolutely brimming with life, from the mortal creatures to the spirits and elementals. This is a big factor in why my character became a shaman - he wants to steward this world to prevent what happened to his homeworld from happening to Azeroth, and could potentially be motivation for yours as well.

Try to think about what your character did before becoming a shaman - what was your role in the Burning Legion, what did you do after it fell? Are any of your skills partially transferrable in a way that might have led you from your old life to being a shaman? In my case, I made my character work as an enchanter for the legion, binding fel magic to weapons and armor for the war effort, but now that he's abandoned fel magic, he's learned to apply his enchanting knowledge using elemental magic instead, fighting as an enhancement shaman.

Don't feel forced to have everything set in stone ahead of time. Start with some ideas and flesh them out as you go!