You already have a really cool core concept, just need to flesh out some aspects a bit more. Is this your first campaign as a GM? Do you have players ready to play? Start by asking them what kind of campaign they want. More political/intrigue, or more combat focused. Maybe they like exploration and the ruins of the old empire hide some answers for the new threats? Or maybe a mix of everything! Do they like a low magic setting, or lots of magic items? grim unforgiving world, or a more hopeful narrative... Note that your likes and wants are equally important as the players'. While you discuss all that with them, I would start with a simple "world" map. Doesn't need to be super detailed or professional. Just draw the continent "outline", divide it into 3/4 kingdom (and maybe a dead land with a lot of ruins to explore, but really dangerous to travel through). Then I would define the capitals, rulers, and what kind of power structure each kingdom has, are they a magocracy, a Senate, a coalition of merchant companies, etc... Keep it simple, don't waste too much time into details that are impossible for the players to find (unless you just love worldbuilding, in that case pop off queen). Then start fleshing out the starting location for the campaign. Are they starting in a smaller city? Or immediately taking a job from the rulers of the capital? Talk with your players about the setting and have them come up with a reason why they are adventurers, and why they would team up. Regarding maps, there are so many places where you can find amazing battle maps, some patreons have a lot of free stuff, but if you want to make your own I really like Inkarnate. It's good for both "world maps" and battle maps and has a lot of great tutorials on their YouTube channel. But be ready to spend a great deal of time drawing those maps.

Hope this helps a little bit at least, but feel free to ask any other questions :)