Worldbuilding 101

Only The Basics!

Start local. A sketch map of a single location and a mental image of the basic culture, climate, and technology can be the basis for a rich and rewarding campaign. The rest of the setting can come as needed and your players can assist in the creation. Listen and learn, referee!

Having a setting for your game is a necessary element of running any tabletop role-playing game. Whether implied by the parameters of the rule system or fleshed out with maps and illustrations and lengthy paragraphs of lore, the setting is the stage on which player characters act out their story. 

The easiest is to set your game in the "real" world such as the New England region of the United States circa 1920s. Of course an anachronism from recorded history may cause wrinkles - which you can explain through the supernatural if you choose. Borrowing a setting from science fiction or fantasy literature has many of the same potential advantages and pitfalls that exists with a historic or modern day reality setting.

Drawing inspiration from history or certain select fictional works while changing the names to protect your anachronisms offers even more advantages than a "sticking to the facts" approach. Having a solid sense of setting details that you haven't given too much preparation effort to, but will invariably come up at the table, is an invaluable benefit of drawing upon one or more sources of inspiration. Names are immensely useful in this effort as they conjure memories and suggest there is something familiar based on the name. A shared image carefully selected from an online search can also go a far way toward suggesting many aspects about the the fictional setting.

At the beginning of play, I have often found it sufficient to describe my game's setting as "rather like Middle-earth, only different" or some similar comparison. The mere mention of something like Middle-earth, with which most people today have at least a passing familiarity, will bring to mind images and shared assumptions which in turn by-passes the need for going into a long preamble describing various elements of the setting and allows us to get right into the action involving our characters. The comparison to a known world establishes enough familiarity for the players to feel like they can make logical decisions for their PCs. Certain expectations and assumptions in the player's mind simultaneously makes the game setting seem more real while leaving the question of "how is it different?" to be answered through play. Having no point or reference, no prior assumptions about the setting can be paralyzing to players as they feel overwhelmed by unknowns. This is obviously to be avoided (unless total disorientation and confusion is your goal and I can't imagine too many players I know going in for that!).

The addition of an extinct ancient culture (human or not) can add a lot of possibility to the setting and present many adventures waiting for intrepid adventurers. An in-land sea (or large lake) is another worldbuilding feature that I find brings a setting depth and additional opportunities for adventure. And don't forget those rumors. They are important to establish ideas in the players' minds and get them going down the road to adventure.
When starting small and adding details as needed, it is essential to take notes and record setting facts as they emerge. The existence of a large urban settlement, the worship of a named deity, a powerful NPC - all should be recorded and remembered for use in future sessions as consistency lends to believability and this is how the game world starts to feel like a real thing. If something does get forgotten, or if as referee we occasionally change our mind and decide to change something, the infrequent inconsistency that results can be described as an "inaccurate rumor" which turns out to be false, but perhaps commonly believed. 

Mysteries add depth to a game setting and I strongly recommend not explaining everything. When a player asks how something works, or if a certain thing is "true" about the world, I frequently respond by questioning them, "What does your character think?" Real people have questions. Real folks don't always know how something works or why things are the way they are. Real life contains lots of unsolved mysteries. Make your fictional setting seem real by leaving many things unexplained.

Reuse the known. By returning to a previously visited location that place starts to feel permanent. By encountering the same individual or creature again and again, the fictional world becomes more predictable and familiar. As a player it is easier to become engaged with and immersed in a fictional world that is familiar and somewhat predictable. Achieving this state at the game table presents the referee with an opportunity to genuinely surprise the players when the familiar and predictable takes a turn.