For the painted illusion of scratches it's really very simple at heart, you just paint a dark line with a lighter line immediately underneath it. Dents are somewhat similar but with softer edges basically - use reference pics, try to paint what you see. There are lots of prior threads on this I'm sure; here's a good one as a starting point,
Painting realistic scratches.
To physically make a dent in a casting there are a number of techniques you can use (including a hot needle or similar for plastic kits) but it's probably safest and most controllable to cut/carve with a sharp hobby knife. I like to use the rounded blades for this kind of thing, the equivalent of an X-Acto #10, rather than the normal triangular blade with a sharp tip but experiment, see what you prefer. If you need to smooth the edges of a carved dent out a little once you're done cutting then fine steel wool is good; 0000 grade if you can get it.
This is the kind of thing where if you haven't done it before probably a good idea to practice on a test piece rather than directly on an expensive mini - you can even practice on sprues to get a feel for it.
Einion