Hello. Experienced self-harmer here. Behind your knee is dangerous, as your popliteal artery is there (as far as I'm aware). This is especially dangerous if you're cutting yourself in volatile emotional states. In these states, you're more likely to cut deeper than you intend to. Spots like these also have thinner skin, so you'll go much deeper without intending to, which *sounds* nice, but if its your first time its much safer to go for spots like your thighs or arms. Avoid the back of your leg in general, as a lot of blood is supplied here, leading to a rather large artery.
My personal two rules of thumb are this: if a cut begins to spurt blood, please go to the hospital. If your cut begins to bleed more heavily in time with the beating of your heart, go to the hospital.
Granted, I am not a medical professional, so please take my advice with a grain of salt.
I still cut on occasion, but personally I have around 200~ scars on my upper thigh, and about 10 on my upper left shoulder.