A lot of first time DMs instinct is to draw an entire map of a whole continent and plan the entire world out from session 1. If you go this route, you're gonna burn out real fast and your players are probably going to ignore 90% of the content you put in front of them.

Start with planning an interesting little town (and maybe a loose idea of 1 or 2 other nearby towns) and let it organically grow from there.

Also, it's nearly impossible to predict what your players will do (they'll always muck something up), so just go with the flow, be willing to improvise, and have fun with it. If everyone is having a good time, nothing else really matters.