Hmm... What I do when I suddenly have something I want to play is I start the process of figuring out more details of what I want. Maybe I want a spooky RP about a hemophobic vampire and her med student roomie who find a corpse in the vampire's bed, but then the corpse speaks! I don't know where to go from there, but that's ok, because that gives my partner and I more room to plan something together.
So, then I take this slightly-more-developed idea, and I start looking for a partner. The best way to do that on Iwaku is to go to the
Partner Connections forum and post what you'd like.
Here is a guide I wrote on that subject that may help a little, and if it doesn't, it links to other resources that address the same topic.
From there, I keep it bumped with relevant information ("I'm still looking!" or "I added a new idea!"—bumps that don't provide informative content are considered spam on Iwaku) until I find a partner and we get talking.
Alternatively, you can make a thread in the one on one forum itself that has the starting post, and give it the thread label "First Response", which means that the RP will be between you and whoever first responds to the thread.
Experimentation is key to figuring out what you like, because there is no One True Way to play: except that you need to remember that RP is about playing. Have fun with it, first and foremost! If you aren't having fun, talk to your partner and see what can be done, or if the RP needs to
end. If the RP dies, there is no shame in it, but by talking it over with your partner first, you may find that you've acquired someone willing to try a different story with you. =D
If you want to try group RP, I can't help you out as much, but give a browse to the
Group Brainstorming & RP Search and
Group RP Signups forums and see what bites you. =D