Read the RP categories thread first so that you don't inadvertantly post an RP in a full category. You should also read the other stickies, as I have tips on RP starting in them. Finally, you need to make sure you have at least one other person to RP with. Other people may join after you start it, but you need to have at least one person at the beginning.
Your best bet is to pick something you're interested, then go on from there. You're interested in Dr Who and Torchwood; you could try doing an RP based on either of them. Just make sure that you describe each show's characters and plot well, so that anyone who doesn't watch it can still RP. For an example of this, read this RP's first page: Condemned: Sonic Destruction
I've never played that game in my life; however, due to Wolfie's excellent description, that didn't matter.
You should also post a plot paragraph at least as long as hers, giving any players something to start from; and also make your preliminary paragraph as long as hers. Don't just post a few sentences; that's what a lot of people do and it's a bad idea, as your post is the first post and the one that all players have to start from. If your RP is based on a TV show or movie, you will also have to pay attention to canon; ie make sure that no-one either plays a canon character out of character, or does something that doesn't exist in the show. For example, in a Dr Who RP, making someone be more powerful than the Doctor.
If you picked something on those shows, that would be Sci-fi; you'd have to PM me your first post so I could see if it would work before I opened up a new category.