My general campaign guidelines:
Start small. Whether it's a village in the wilderness a neighborhood in a large city, you only need broad brushstrokes at first. I typically have 5-10 NPCs figured out ahead of time, with some generic notes about other important individuals (mayor, king and so on) that include simple sentence or two like: Mayor Alfredo is well known for being larger than life, with a ruddy complexion, wide girth and a fondness for elaborate galas.
Is Mayor Alfredo a good guy? Bad guy? I don't know yet and may never know depending on what happens.
Drop hints for future plot hooks and pay attention to what your players take an interest in. Have the group overhear a conversation about how the mayor is corrupt or that there are more refugees coming in to town from the south than is normal.
Have broad overarching plot lines, but only as a general guideline. Feel free to change your BBEG's motivation at the drop of a hat as long as it doesn't conflict with pre-established dogma (or if it does find a way to explain why the party was misinformed).
I typically have an opposing faction or two that the group needs to worry about at any time. The factions may or may not be minions of some other group. I figure out broad goals and think about what they are trying to accomplish. For example if the group doesn't follow up on the refugees, then an orc tribe will continue to grow in power to the south. They may or may not be a direct threat to the PCs but they will have an effect on the region.
Keep lists of names. Taverns, businesses, NPCs. Just have a list handy and when the group is looking for a tavern check your list and find that you have "The Grumpy Dog" listed. When they interact with the bartender grab the name "Olaf Signurdson" who directs them to "The Crazy Gnome" trading company.
There are a ton of name / town generators out there. For example one of my favorite for names is Behind the Name because it generates realistic names and gives you the definition of the name if you want.
I also try to end sessions giving people options of where to go next. Give them 2-4 options of what leads/rumors they want to track down and ask them to tell you which one they will pursue. That gives them an opportunity to direct the campaign while also giving me time to prep for the next session.
Last, but not least, listen to your players don't be afraid to improvise. Some of my best ideas were those that played into the PC's fears or speculation. If someone says "What if the mayor is really a doppleganger?" I think about whether it would be a fun plot hook. I've had entire adventure arcs start that way.
Good luck, and have fun.
) but there is definitely a purpose to their adventuring. Pluck dungeons and encounters from wherever (published adventures, DMs Guild, your head 
