Easiest, most straightforward method is to just open your Applications folder in Finder, switch to List View, and look at the size values. But that doesn't catch the full usage of an app, since they all keep stuff in your Library folder (specifically, the Preferences, Application Support, Caches, and—for Gatekeeper-safe apps—Containers subfolders) and occasionally your Documents folder (usually this is just for files you created in the app). If you have hidden files displayed, you can go through there looking as well.
But that takes ages, so your best bet is to use the command line or an app. I like OmniDiskSweeper, which goes through your hard drive measuring how much space each file and folder takes up. You can work through the hierarchy of that to find what's using lots of space. If something you don't recognise uses a huge amount, feel free to ask us what it is and whether it's safe to delete.
As for how much space to keep free, a good rule of thumb is 10% of your hard-drive capacity. So if it's 500GB, try to keep around 50GB free. It's less an issue as capacity goes up, though—as long as you have a few spare gigs for swap memory and caching, you shouldn't hit any major slowdowns or snags. You can safely go a lot further than you have already, before you really need to start trimming back.