Question

How to start a sentence off when describing someone?

Answer 1

There are many ways to describe someone, it's generally limited by your imagination and knowlegde of a language :) However there's a rule that when describing someone, start from the top. So firstly you describe head (or eyes, nose, mouth, hat, glasses etc., depending on how many details you want to describe), then torso (arms, chest, jacket, t-shirt etc.) and legs (analogically).
 
I know it's not exactly what you meant but I hope it will give you some clue :)

Answer 2

Well it really depends.

If you are describing a 12 year old girl, with brown hair and brown eyes it could go something like this:

"The young, twelve year old girl, had long brown curls that fell to her shoulders. Her chocolate brown eyes were as dark as her hair and stood out on her pale, freckled skin. She was - in short - lovely." 

If you are describing... let's say... a 16 year old boy with black hair and blue eyes it could go something like this:

"The young man turned his crystal blue eyes in my direction and I looked away quickly so as not to be caught staring. "How old are you?" He asked. "Sixteen," I replied timidly, twirling my long hair around my finger. "Me too," He replied, flicking his shoulder length black hair out of his eyes."