Basically, a twinneh can be anyone with whom you have ever come in eye contact. It could be your best friend or someone you've known for three minutes. It has positive connotations. It is commonly used on myspace by really trendy kids/scenesters/emofucks.
To truly proclaim someone as your twinnehh, you must make a sign saying, "insert name here is my twinnehh!!," preferrably hand-written. You must then take an emo picture of yourself making a weird expression and/or hand gestures. Then post this picture wherever you can on the internet, so everyone knows.

When you eat dirt for the first time and leave your family to build a shack in the woods