I was just wondering, how do forensics people find out about DNA or fingerprints?

Blood samples and things like that

Well, when they take a sample of DNA (like at crime scene), they can match it back to someone with the same DNA. They know who's it was as everyone has different DNA. Family members have simlilar DNA, but not the exact same. Yet, two people can have the same DNA if they're identical siblings (rather it's twins, triplets, etc.). This is because it splits into two (or three or so), so they have the same DNA. However, they do NOT have matching fingerprints. So if you ever have a set of identical siblings that are part of crime (or one is), and their fingerprints and DNA are there, then take fingerprint samples from both and it'll say which one did it. Fingerprints are different for every human (even identical siblings). And of course if police find your DNA or fingerprints at the crime scene, they can place you there (or by other things to). But that's basically how it works.