Take it from someone who has been doing this for over 16 years, and has worked with some of the greats and even got a Shadowrun mission published: GMing is hard. Like, I've run six campaigns in the last four years, and three fell apart due to a lack of ideas. (Two are currently running, and the first ran for two whole years before everyone finally decided we wanted to do something else.) That said, here are some tips:
*Draw from your setting's established lore. If your setting is homebrew you've got a bit of work cut out for you, but don't ever be afraid to borrow from other settings. I once ran a Dragonlance game that involved an evil wizard who fled into space using a magical airship called a Spelljammer that I stole from another D&D setting.
*Get your players to give you ideas, but be subtle about it. Ask what some of their favorite fantasy tropes are, or what they'd be interested in the future. Have them come up with npcs their characters know, who you can then use to draw them into the story.
*Don't be afraid to make up utter nonsense on the fly, especially if the players are going in an unexpected direction. If it was good enough for Gygax it's good enough for you.
*Never involve flumphs, your players will hate you.

You could have a plot prompt, like "A dragon soars overhead, burning a village" or "You randomly get frozen by an ice wizard and thaw out five years later, in a different world" sort of thing. I've done many RPs before, and they usually happen like that. Odd jobs, then *Bam!* Big Bad plot gets shoved in your face! Try your own thing. You get the hang of things quite quickly.

If the string of unconnected events needs to be connected, you could always say the Big Bad of the unconnected events are working for the Big Bad of the campaign.

If not, then chalk it up to the adventurers just having a rough start and finding their feet doing low-key adventurer stuff.

And for future events, find ways to tie them in to the ultimate Big Bad. :)

Easy as that!