Hi I'm 12, have some pubic hair and when I get a erection they are 4.5 inches. Sometimes after an erection a drop of white liquid comes out of the penis. I have some super fine armpit hairs. People I know says that my voice is deeper, but I don't see any Adam's apple sticking out yet. Did I reach puberty yet? And what is the drop of white liquid that comes out my penis after an erection?
Puberty is the time when you begin to change, though some people will loosely use the term to apply to the entire period of change, which is more properly called adolescence. So, in answer to your first question, you are long past puberty, by several years actually. From your description, I would guess that you are near the beginning of stage 3 in development. You can get a better estimate by using the Tanner Stage Calculator for Boys.
Drops of clear liquid that feels very slick which comes out when you are sexually aroused is from the Cowper's glands. It is call pre-ejaculate or in slang pre-cum. It's purpose is to clear the urethra (the tube in your penis) in preparation for ejaculation. The Cowper's glands mature enough to produce pre-ejaculate at different times in different boys, but for most boys it happens in stage 4 of development.
Whitish liquid when you are sexually aroused is generally from the seminal vesicles or the prostate gland. It feels sticky when touched. It is part of what makes semen, but since you are not fully mature in that area it is only functioning a little bit. You're getting a few dribbles, but your body isn't mature enough to actually produce a quantity of semen and be able to ejaculate yet. Typically that happens in stage 4 as well, though some boys gain the ability to ejaculate in stage 2.