A couple of things to keep in mind is that you should prepare ample time for your sessions and that you shouldn't be afraid to set ground rules, even if you're playing with friends.
I've had games where the pacing crawled to a halt for sessions without end. There was one game where the DM didn't want to interrupt chatter, so the party spent two whole sessions in a single combat. Additionally, prepping for each session took hours. No one kept their sheets up to date and had to ask questions about every skill & interaction. In other cases some people have wanted to Roleplay as if it were a single-player RPG, leading the (unwilling) party and diverting attention away from others and to themselves.
This isn't to dissuade you, rather playing with friends is incredibly fun & satisfying. However, a lot of newer DMs are afraid of speaking up and end-up leaving their games to die without reaching the fun parts.
As for coming up for a setting or story, I always keep this quote from Chris Avellone in mind:
"The best technique I ever learned was to try and figure out what each character's power fantasy is. Do they want to be the strongest hero? Do they want to be the cleverest guy? Do they want to be like John Constantine? Do they want to be like the Gray Mouser? You figure out what their power fantasy is, and you design the adventure so there's always an element where they can shine, and then they walk away from that event going, 'You know what? I contributed. I felt awesome, and I helped the whole group achieve their objective.'"