Well the easiest thing to do is grab some paper and just sketch out the world by hand - that way you always have the general layout to look at. It does not have to be precise just basic blobs and basic terrain (like upside down Vs for mountains, some squiggly rivers, quick forests) and you can put some fantastical races there for future reference like mermen or cat-people or something. Then pick one area of one continent where you want to detail and go to it. Keeping the other continents non-developed leaves room for further adventures later on as well as keeping the main continent sketchy at best. This way you never get lost in your writing but things are not nailed down either.