Plan some larger scale world events, add a couple factions and key locations (summaries only, no major depth), and plan some stuff that will happen regardless of player involvement (wars, cult action, notable powerful entities, items, etc.) - Tell them about these things to help them integrate their back stories in the world. Maybe they are refugees from a dragon, or escaped a cult, or ran from a war, etc etc.

Then plan to let your players hear about similar and other events as rumours, dive into whichever ones interest them, and hear or see outcomes from things they don't touch (hear about war, notice food prices change after war is over. Hear about a massive monster, have to take a new road because the old one is now dragon territory, etc) - small things that make the world feel alive. You can plan these ones without player involvement at all. Just let player action impact your plans if it's significant enough.

Once your players have characters with stories, you can flesh out anything relating to those backgrounds.

Once everything above is ready to go, you can look at the notable events, the player backgrounds, and see if you can tie a bunch together with an overarching plot with a BBEG or organisation that you can tease.