Welcome to the forums! Enjoy your stay and have fun! Remember to praise Rarity! And for the cutie mark, go to settings at the top right and then 'Edit Profile' on the left. There will then be a field to enter in a link to your cutie mark.
Silly pony, you can't just put a cutie mark on your flank. You need to find out what your special talent is first, then it should appear on its own. All kidding aside, This works. Alternatively, you can just go to your profile, click the About Me tab, click the pencil that's right next to "Cutie Mark" and paste the URL. Where to upload the image; any image hosting site will do, but I think most of us use Imageshack. Of course, welcome to Everypony, TunderHugs! I hope you enjoy your stay with us!
Hi Tunderhooves! Welcome to our herd! Hope all that above helps. If not, don't be bashful, ask more detailed questions. I, for one, am very curious about your cutie mark. I'm not sure what a "tunder" is, so I'm quite curious to see it. In the meantime, post ponies, listen to EPR, make friends, and above all - have fun!