You need a germ of an idea of what you campaign setting will be. It could be a map of the wilderness, it could be a city or a dungeon or a world or an idea for a story in which you players will be taking place. I had an idea for Midlands, and the nature of the world dovetailed neatly into the arch quest for the campaign. Beyond that I started with a Barony, that will be the initial base of operations for players for a while, after that I figured out roughly what lay the North, East, South and West of the Blacklands Barony. I did the geography for the Barony, and figured out various travel times between different locales for future play. Finally I came to the point where I needed to flesh out a lot in an instant. There were warrior houses, thieves guilds, churches, trade guilds and a few other players in the Barony, with the Baron skillfully maintaining harmony among the diverse interests. I needed to flesh those out and stumbled upon the concept Excel Spreadsheet for design.
Across the top of the spreadsheet I put the names of the warrior houses, which train and lead the various little armies of the Barony. These were Sele Baar, Sele Klawu, Sele Heeglah, Domo Nattricks, and Sippe Kottah.
In the left most column I put the criteria along which the different houses will be compared - type of livery, how their warriors are armed, what kinds of troops and how many each, how each warrior develops, for instance, the traditional houses Baar, Klawu and Heeglah are more or less traditional Anglo-Saxon style warriors, but the Snakes (House Nattricks), their knights can fight in pitch dark and are fighter-thieves, while Sippe Kottah are fencing masters who fight with a main and a companion sword, that is their school. Other conceptual criteria would be each house's main source of revenue, some collect rents, others run women and prostitutes, one loots dungeons and trades in magic items, etc; where their men hang out, and what if any, have ties with the underworld. Couple of Houses themselves run their thieves' guilds, as was historic, while some are corrupted by their thieves, while still others are at odds with them.
This Spreadsheet keeps expanding with time like a sponge, and you can have others, contrasting and comparing various features of your setting. One of the most important things is to be able to produce a combat encounter on the spot, because the minute the players want to take violent action and the DM is not ready to roll the D20's, the game either stops or turns into a bullshit session. To avoid that, flesh out the details for armor, weapons, ability and HP for the men at arms, various guards and henchmen which fight and protect various interests in your sandbox. Once you have those in place, it will become much easier to pull out hostile encounters on the spot, as the situation calls for them.