My first time running a homebrew campaign

I'm preparing a homebrew campaign/module to play with friends, it will be a long campaign, in the style of Tyranny of Dragons, Out of the Abyss or Turn of Fortune's Wheel, the idea is to start at level 5 and finish at level 20 (maybe even later, but I don't think so)

I have a more or less clear idea of ​​what the second half would be, the main villains and their objectives are clear, but the beginning, and basically everything that happens before the players discover the seriousness of the situation they are getting into, I don't really know how to present it/design it, I know that on several occasions a messenger from an entity that is currently anonymous (spoiler: Bahamut) will give them missions to test them, but that would be shortly before they find out the truth, above all the problem is that I don't know how to start, what they can do to get the attention of that entity and how to make those missions not feel like a drag, besides, I would like to connect the characters' backgrounds/motivations to that first part of the adventure, surely relating it to the tests they will have to take

So here I am, looking for any advice that will help this poor newbie

By the way, to level up we will use the milestone system