If it actually came out of a dumpster, you might want to have someone electronics-savvy open it up and check for any "shipping damage" before applying power. Leaking batteries are trouble, but loose connectors, or conductive parts rattling around the circuitry inside are more likely to cause immediate damage when the juice is applied.
If it's a 3000T, the keylock on the front is a standard feature of old computer towers (the T would be for the 'tower' case), both IBM-compatibles and not, to allow for 'securing' the system (disabling the keyboard, maybe the Amiga incarnation properly disabled the whole machine) in an office environment. Chances are it's unlocked, and if it's not, it'll be ridiculously easy to bypass (whether by shorting two wires, or just finding a key -- pretty much every case ever made used the same style, and they weren't cut individually or anything like that). Doesn't have anything to do with keeping the case from opening.
Once you've made sure it's free of roach nests, mouse poop, or anything like that, you can plug it all together and see what happens. The Amiga has a "color-code" system for serious hardware problems, so if something major is wrong, the screen will come up in a solid color (like, say, red) and we'll be able to tell you what to do. If everything's okay, and it has a hard drive (that hasn't been damaged), the machine will boot; if it doesn't, you'll get a nice image telling you to insert a floppy, which you can do if the machine came with any -- try a game. (Please blow out the disk slot with some canned air, first!)
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If you're wondering what you've got in more general terms -- what you can do with it after it runs -- the performance is on par with, oh.. I'll be killed both ways for this, but let's say a "486," in terms of overall ability. Of course, the Amiga is a whooole lot more fun. :-D
"Desktop publishing," graphics work, analog (VHS-style) video, word processing, and what's now a slightly oldschool form of digital music (the Amiga was basically the first platform where you could work with "samples" of real recordings at all) are all readily possible no matter what software came installed. Internet connectivity and web browsing can be done, but it's a little bit fiddly for a novice to set up (somewhat similar to an older Mac in that aspect, slightly more complicated in that you'll need to track down the right software yourself... and at least Internet Service Providers have heard of Macs).
Beyond the Amiga OS, you can also run various flavors of "UNIX" on it, which was rather a big deal before "Linux" hit it big ("Wow, it's like having your own personal supercomputer!")... Of course, these days you can run Linux on it too. (Something called "NetBSD" is probably better if you'd want to, but that's a story for later, and all of that requires at least one serious nerd to get going.)
...and speaking of Macs, there were/are also ways to "emulate" the older Mac OS on it, but most of that software is now so out of date that it's not worth the trouble. As you'll find out, Apple may have been prettier at times, but the "miggy" was always the more capable machine. ;-)
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Careful un-knotting that mouse, it's not something you can replace at the local computer dealer, though there are still places to order spares.
