With regards to using Flitterheart as a character:
Theres no reason whatsoever that you wouldn't be allowed to roleplay as Flitterheart on this website: you might have to apply to use her if you are using the Equestrian Empire forum, but for the normal RP forums you are free to use whoever you want - save for characters created by other people.
I would strongly advise creating a character your own however - as this means that you can create every aspect of your character. you are free to use whatever picture you want for your character - just remember that if its drawn be someone else, you'll need their permission!
You can use the link below to get the the character database: http://mlpforums.com/page/roleplay-characters
From here, click the button that says 'Add Character' to bring up the template to create your own.
Again, you can use existing characters but I strongly recommend creating one of your own. Making a character page isn't compulsory, but if you do then you have a permanent record of your character that you can use to show others everything they need to know whenever they need to see it - and a lot of roleplays will require you to have a link to a character that you can provide.
For example, here is one of my own:
http://mlpforums.com/page/roleplay-characters/_/cinderblock-r3563
Then, it's just a case of finding a roleplay to join!
The Elusive Cinder's post in Can I make Flitterheart? was marked as the answer