Dermablend, Kat Von D Lock It Concealer, and Glamoflauge can all be excellent ways to cover up scars. Dermablend is waterproof and covers very well, although if your scars are raised, it may be more difficult to hide if you use it. Kat Von D works very well, is waterproof, and lasts all day, and is especially good with oily skin, but it's expensive. Glamoflauge is inexpensive, it's waterproof and it lasts all day, but it only comes in three colours and can be very cakey.
If you don't already know the best way to cover them, these two videos will show you how to hide your self-harm scars. By the way, don't put make-up on open wounds, because it won't stick and it can cause infections. If the self-harm scars are still healing, you'll have to wear things that will cover them up, like jeans, hoodies, and jewelry bracelets. (Consider plain but big stuff that won't draw people's attention but will still cover the scars.)
In the meantime, Mederma, cocoa butter, Vitamin E oil, pure aloe vera, and tea tree oil all work well to help reduce the appearance of scars. (Tea tree oil also works especially well on acne, but be warned, if you have particularly sensitive skin, it can burn.)
Hope this helps!