Top of thighs is a common place. You can also use bracelets and makeup to conceal scars and when it's hot wear long sleeved linen shirts. I suppose the upper arm near where the shoulder would also do but I think it's a bit too fleshy. Lower legs near the ankle and feet are a good place and if you're feeling really shitty, I've heard online about cutting on the stomach.
I will warn you though: self harm is an addiction due to the way dopamine works and I wouldn't recommend it unless you find enjoyment out of it because those scars are gonna stay, especially if you go deep. It'll affect how you think, how you see yourself, how you go about your day. It's not something you do every now and then: it's a blood drenched force that'll make you go further and further into mental health decline (if you're doing it for the most common reasons that pop up in those PSAs). People won't look at you the same once they know. You won't look at yourself the same and if this goes on long enough and if you have enough determination: then you'll end up with a trip to A and E followed by the mental hospital once you hit an artery or really fucked up a nerve. If you're gonna harm yourself, don't go the cutting method (you can get tetanus and stuff aswell), sit in a boiling hot bath or just punch yourself until you're bruised. At least then the damage won't be as bad.