u/Oneiropticon

I'm totally clueless, only beginning to dabble to adjust a 3d print file a bit, and the tutorials are a little like drinking from a firehose, but I've got a problem that I think should be simple. I'm opening an STL file, and I can move it around fine, but any time I try to select anything, the entire project selects, but nothing I can find actually changes anything but the perspective. Am I barking up the wrong tree entirely? Am I missing something painfully obvious?