Step One:
Creating your character(s) is key to success. Most people prefer roleplaying with one male and one female character each. What I find the easiest is to find an image of someone (you may use tumblr or google for this).
For example, you may pick to use Lady GaGa as the appearance of your character. Now you get to choose a name and what their personality is like.

~example 
Name: Nikki Brown
Personality(may also be classed as 'Other'): She is a very shy, outspoken girl. Tends to spend most of her time lost in a good novel or her music. Most people compare her to a mouse due to how quiet she is, even when speaking. However she has a beautiful singing voice but is too afraid to sing in public.


Step Two:
Now that you have your characters basic information sorted you need to know how to roleplay. There aren't really any rules on the best way to write your piece about your characters. The best thing to do is use your imagination but stay realistic to the theme. 
(For example; if the plot/theme of your roleplay is a highschool scenario, it would be unrealistic to have vampires there. but if the theme is supernatural highschool, then it would be acceptable for vampires to be there.)
The best way to make your post for your characters is to write as if you were writing a short story or a book. People usually write in third person.

~example
Nikki could hear the muffled piano and vibrations from her rather smashed up iPhone that was somewhere under her two pillows. It was clearly time to wake up for class but as usual on a Monday morning, she didn't want to move. So she stretched out her hand and felt around for the lock button which forced the alarm to go onto snooze mode. Allowing her an extra eight minutes before it would bother her again.

QUICK TIP: Try to be as detailed as you possibly can be. As you can see in the example I spoke about the situation of waking up to an alarm clock and putting it on snooze. But I added a bit more depth to that action. This will improve your roleplay and gives your reader something interesting to work with.

Step Three:
Picking a plot can sometimes be the hardest thing to do. Personally, I prefer going with what I call a "basic" theme. Examples of these are high schools, shared apartments, hotel and work place. They are themes that you and your roleplay buddy/buddies can expand on with your own ideas as the story line progresses. 
Others sometimes prefer detailed plots to start with. These are almost like the blurb on the back of a novel. Something that summarises the storyline. Examples of these can usually be found by googling or on peoples weeblys. Or if you have a plot you'd like then write it up and use it! Roleplaying is all about imagination and having fun! You could also base the plot off of one of your favourite TV shows, movies or books.

Step Four:

Now all you need to do is find yourself people to roleplay with! The best way to do this is rock on over to the RPG section of the forums and post there! You can either start a roleplay yourself and wait for people to join OR you can start a forum searching for roleplay buddies. The world is your oyster! I hope to see you there!