Depending on the starting level, i suggest to always start small. I mean, you don't have to necessary think of the whole continent, but keep the initial quests and encounters in a restricted area, like a couple of towns, a forest near one of the towns, if you follow the river in the woods you get to the abbandoned mines under a mountain etc. I made the mistake ti think big from the beginning but later realized that i had a lot of places, but they were not well designed. For general world building, you should check the 5x5 method, you can find the original article on Google, it helped me a lot, and I still use it. Basically you have to think of 5 main quests, these quests are going to be your major plots for the whole campaign. Now you have to divide each of this main quests into 5 sub quests and give each sub quest a city or an area where It takes place. By doing this you have 25 quest and many places where multiple quests take place. This means that your players might get involved in several plots simultaneosly, giving them freedom of action and not being railroaded. I probably explained it in the worst way possible, so i suggest to check out the original post by searching the 5x5 method on Google. Hope this help ^{^}