I, too, am much more given to worldbuilding than I am plotting, but you should regard your strength in worldbuilding as just that - a strength. Depending upon what your goal for the RP is, it may be sufficient, but in that case you'll want a dedicated group ready to advance the story and craft plots of their own on a regular basis. I've had this before - it's awesome to see a bunch of players take the detailed world you've given them and just
run with it for fifty pages even with no real predetermined plot. It's kind of a rarity in my experience, though. Not everyone is going to be able to do that, and it's not something that should be expected of them, either. Most RPs will benefit from GM-forged direction. Still, giving your players a solid background in the world can be step one, in my eyes. They'll have a framework to inspire them and work off of, and that will help them work your plot as well as their own when they're inclined.
A lot of good has been said here already. I agree with Tea that some of the things you can do to help general writer's block can be useful here. You could freewrite, do brainstorming exercises, try writing prompts, and so forth. The goal is to get a lot of ideas on paper. Then, you can throw out (or at last put away for later) everything you don't need, picking out only the things you think will work. I would add that making use of the unexpected can be helpful. Once you have your basic plot ideas down, shake them up and see what's rolling around inside. This is where exercises and brainstorming can come in handy again. Just take something (and if you're stuck as to what, there are a number of places to get writing prompts and random twists online) and throw it into your idea, play with it, and see what happens - you might surprise yourself. If it doesn't work, you can toss it out with no harm done, but it just might give you something to build on.
LSunday's advice about partnering is also good if it's your style. I love working with others from the ground up; the mixture of ideas and perspectives can really help plotting along. If you can't think of something, someone else might.
Shanatos's emphasis on conflict and goals is something to pay close attention to. These incredibly important in general writing, but I feel that they become even more important in roleplay because of the collaborative nature. Everyone is advancing the story in part; they need things to work off of. If no conflict is presented, the game can stagnate. If everyone is standing around with no motivation, the game can stagnate. Try to present opportunities for conflict within your plot, and try to give characters things to want, reasons to want them.
Another thing you can do is work off of archetypes. It has often been said that there are really only a few different types of plot, though exactly how many gets divided differently (you can see a few different systems
here, and I'll provide another link on the "36 plots"). Whether you agree with this or not, they definitely present a good place to mine for frameworks and inspiration.
This is a really good look at what the "36 plots" are and how they can be used in RP. And
here is a really cool list of plot types that the author created based on lots of published adventures for various tabletop RPG systems (fear not, they work with plotting freeform games equally well). The key with these is not to rehash a cliche but to look at these frameworks and figure out how they can interact with your setting, your characters, and your players' characters - how you can use them to tell an interesting story with everything you have to work with. Don't think you have to stick to just one, either, or follow them exactly. Pick and choose whatever you think works. It can also be helpful to think in terms of story arcs since your game is going to be continuing on - you can take one big plot and try to think of lots of little situations to help advance it. Additionally, always remember to leave wiggle room. You want to stick to plots that your players can work with, and you don't want to railroad them.