How to plan and build a World and Campaign?

I have been dming for various groups for about 3-4 years (on and off). 4 friends and myself want to start up a long-term DnD game, with me as the DM, and I really want to not use a prewritten world and campaign, like all my previous campaigns. I want to build the world and the campaign from the ground up.  
The campaign isn’t starting for about 3-4 months, so I have plenty of time. I also want this to be my best work and make it fun to play. 
I am just wondering how to go about this? What do I plan first, how do I plan? I have vague ideas of what I want the campaign and world to be about, but do I start with a map? A starting town? Or a starting session? World of Campaign first? 

Thanks in advance