To make one you need an art program and a little skill. How much skill depends how good you want it to look. ;)
Or you can contract an artist to do it for you - many of them advertise in the Commerce forum.
To get it onto your posts, you need to edit your profile to include it (using HTML, not BBCode) and then remember to check the box at the bottom when you post (since it doesn't do it automatically at the moment). There's a sig testing thread in the Art Public forum if you aren't sure whether you're doing it right.
"May the Grace of Ara go with you, and His Vengeance be wrought upon your enemies."