Small suggestion from me, echoing what DruSM said, try making an Interest Check for a relatively developed idea.

The other way works as well, listing like "Starter Ideas" for RPs that you might be interested in or general settings you like...
But that tends to work better when you have written with more people so they know who you are, how you write and if you're someone they enjoy writing with.  A good few people here do that but it's only really super useful to someone who has written with you before or knows the quality of your writing.  Like I know a few people who do excellent Elder Scrolls RPs, a few people who do a lot of Slice of Life High School or Supernatural RPs (linked below), and a few people who do Warhammer RPs.

Making an Interest Check for an idea you have partially developed helps indicate to prospective partners what interests you and where your writing is.  If you have an idea for a world or a dilemma that really interests you, and it sounds like you do, then developing it into a more involved Interest Check might help people who stumble upon it become interested in it as well.  That was how I got started writing here, just found an Interest Check that sounded interesting and that was it.  If it's well developed it can serve as an introduction to a world that might be a new take or just something folks havent seen in a while.  If you're sort of at a loss for how to continue it you could always put up an Interest Check with what you have and specify that you're looking for people to RP with and collaboratively make a world with.

It's the difference between "I'm interested in High School Slice of Life RPs" and https://www.roleplayerguild.com/topics/183907-rosefell-high/ooc

Try not to take it too hard if you post an Interest Check and get little to no interest too.  There are a lot of RPs in progress here generally at any one time and a lot of writers don't want to join more than they feel they can handle.  If you happen to make a Murder Mystery RP at a time when there are 3 Murder Mystery RPs underway you might just not find any takers.  

Finally if you just don't seem to be having any luck with starting your own RP sometimes the easiest way to start getting active is to join a story that interests you and start getting to know other writers.  If you join a story that's not 100% your cup of tea but was a weird setting with a character or two you really liked you might find yourself realizing you really like something you didn't think you would ever try.