When beginning a rp one must also not only have a character, and topic in mind but a scenario as well. (Aka what's going to happen, and what your character's going to do.)

Here's an example scene :

In Third person :

Dara walked onto the busy and wet London streets . Dark hair soaked by the rain, though it was half covered by the red umbrella she carried in her right hand . Upon hearing a noise she turned , eyes and ears alert as she looked for the cause of the sound. Finding none, she shrugged , carrying on her way to the little café down the colorful and green alley.

In first person:

(This is how I usually do it:

(Dara)
I walked into the busy and wet London streets. My dark hair soaked by the rain. Though I had it half covered by the red umbrella I carried in my hand. Hearing a noise made me turn and I kept my eyes and ears alert as I glanced around. Trying to figure out the cause to the noise. Finding none, I shrugged. Continuing on my way to the little café down the colorful and green alley.

Roleplay is simply like writing a book . :) Though its with two people and you don't have to write as much.

If you can't figure out a topic or scenario discuss things with your rp partner. He or she might have some good ideas too.