1. Do you have a local bike co-op? If so, I'd start there - you'll be able to get inexpensive parts, and help from someone knowledgeable. The parts probably won't be light or high end, but you could always upgrade them later if for some reason you found it necessary. If you don't have a co-op, I'd try Craigslist or Ebay, but that requires more understanding of what parts you need. For buying new, there's enough of a shortage of parts these days that you'll probably have to poke around various online shops to see who has the parts you want in stock.
2. Search for the bike's model online and you should come up with reviews that are likely to include detailed specs. Bikepedia often has useful information but not much for your bike. Did the frame include the fork and headset? Do you have the bottom bracket (the bearings for the cranks)? Those are pieces you need to get right; pretty much everything else is either standard, easy to swap, or does not need to match a particular spec. Those (headset and bottom bracket) are also pieces you may not be able to install yourself without buying tools you won't need often. If you don't already have the right tools, take the frame to your local shop, ask for help, and pay them to do the installation even if you buy the parts elsewhere.
3. You need the same parts for a flat bar as for a drop bar, except that you need shifters and brake levers that will attach to a flat bar rather than a drop bar. You need to decide what drivetrain you want to use (how many speeds? What brand and model?) and then buy appropriate or compatible shifters, brakes and derailleurs. You'll probably want a long stem since flat handlebars don't add additional length.