When I was a teenager I hurt myself (but not cut myself) at my hands and my arms and sometimes my at my legs, too.
Then, there were years without SI and when I started again last year (cutting this time), I cut only my arms (outer sides and sides of them) because they are easier to reach and - sorry for writing this - but I thought that "this is where people cut themselves". (Yeah, I even did not think about cutting anywhere else
. )
Moreover, you can hide it but sooner or later, people will find out (because of long sleeves and armwarmers even when it is really hot) and I guess that in a way, I WANT people to know what I do. (I am very open about my SIing.)