I first ran an RPG campaign at the tender age of thirteen. My brother and I sat down with my brand-new GURPS 3rd ed. Basic Set (back when it was one book instead of two!). Running a campaign fills me with nostalgia. I’ll spend this series introducing basic concepts and working through obstacles so that anyone can start their own campaign with friends.
Before you progress in planning your campaign, you need to decide how long you plan to make it last and what sort of structure it will have. As John Shaffstall wrote on storyline types, there are only a few basic types to choose from.
Once you have a genre, a few tiny ideas are probably going to pop into your head. Inspiration can come from sources like TVTropes, the Big List of RPG Plots, and the random generators at ChaoticShiny. To speed things up, try what Mike Bourke suggested in Baby Steps In Campaign Setting Design and figure it out one detail at a time.
As you worldbuild, remember the notes and information organization system you settled on early in this series. Keep track of everything you decide and all the ideas you get so that you can create a rich world for your players.