Gondor's posting requirements made me the writer and GM I have been for years,
@Pele Alarion 
. I know AoA looks intimidating. Well, I've been told this rather than actually believing it. Some folks likes to show off what their characters look like so that's why there is an abundance of icons. A lot of participants have much to respond to or let their own vivid imaginations run to their heart's content in solo material here. There's banners signifying the place being written about, they're pretty helpful. You are definitely not a newbie but a seasoned writer and ruler to boot, one of the best which is why I've always wanted you here ⚘. Anyways, how to begin....
It all depends who your Mortals are. Do you know the Silmarillion well? If not that's okay, Cassie and I can talk to you about what each region is like. If you have any humans living in Hithlum and one of them is a soldier, he or she could be a commander living at Barad Eithel, a tower near the mountains of Ered Wethrin overlooking the mighty river Sirion. He or she could be standing guard at predawn watch in the winter. Seeing rivers of flame destroying swaths of the snowy grassland, Ard-galen and elven riders fleeing from the consuming flame. He or she can run to the council chamber where the elven High King Fingolfin (me) and your mortal Chieftain - Hador (Played by me since Turin Ringhun is busy) - have hastily assembled, asking what they should do. You can notice Dwarrow Elf's NPC, Earcolante, an elven cavalry captain, having collapsed with burn wounds who arrived shortly after you entered.
If you're a human of Dorthonion instead, a soldier serving at a hill tower called Mindo Manheren above a pine forest, you can have your warrior asleep but suddenly alerted hearing alarum bells signifying a present danger to escape from. Vacating the building as it's caught on fire, you catch sight of fellow soldiers Nathaniel and Beledor (my NPCs), Margh and Mads (Cassie's NPCs) pulling the elven lord Rincion (My NPC) out of a pile of flaming rubble.
I think that's the best way to begin. If you have civilian mortals I can let you know how to start them too. It'll get easier once you begin writing and following one of the storylines.