I was wondering if you could help me with this: I'm starting to write a book, but I don't really know how to describe the physic of the main character (in 1st person). I heard some techniques like, compare the character with herself when she was young (while watching an old pic), but I don't know if techniques like that look good in the story. Any advice? Cheers.
Your narrator compares herself to others.
I met Sandy at the coffee shop. I towered over her by a full head.
Cheap and simple: Your narrator looks at himself in a mirror.
In the bathroom, I ran a hand through my hair. Still black, just like my dad's, although thinner. He went gray early — nothing for me yet. I had my mom's blue eyes, which was surprising unless you knew I was mostly Welsh.
Straightforward: Your narrator describes himself.
I caught the guys in the bar looking. They had plenty to look at: tall, blonde, green-eyed, still pretty fit. Not a six-pack, maybe, but not a beer belly either.
There are several techniques for doing this, as Lauren Ipsum illustrates, but consider that the first person narrator is also a character and how they narrate the story is part of how their character is formed in the story. So if the narrator describes their physical appearance, the fact that they are doing so says something profound -- and not necessarily very complimentary -- about their character. It may paint them as vain or self obsessed, for instance.
Also consider that by choosing a first person narrator, you are inviting the reader to adopt the narrator as a substitute self -- to see the world through that character's eyes, rather than to observe them directly. Therefore it may not matter much what they look like unless some aspect of their appearance is inescapably relevant to the story.
In short, it may not need doing, and will almost certainly have side effects that you may not want.