Thanks for the invite!
This is one of those systems where I'd say, if you can just buy something (and you can) then just buy it. There is a lot of liability for a company to keep correct attendance; people will get pissed if they don't get paid on time or correctly; the company can get sued if it's incorrect, etc...
But - if you must -
1 - you'll need a desktop app of sorts be it Windows or OS X or Linux to interact with the barcode scanner. A small pc or Mac Mini to place at the scanner location to plug the bar code scanner into - look into an HP Thin Client or similar - something you can hang on a wall or under a desk.
2 - Find a good bar code scanner with a well documented API - make sure the API can be used in VB.net or C# for a pc, Objective-C or Swift for the Mac (I can't speak to linux)
3 - Build your app - in Windows, the easiest choice will be VB.net or C# - both have tons of tutorials and docs online. If this is a 1 off for a company, I'd say run a web server in house and send attendance to that - again, for liability. But if you intend to sell as a product, make your app configurable to send the data to a cloud server.
The server side of things can be as simple or as complex as you want. With PHP / MySQL - probably a lookup table with the name of employee and their barcode.
Then the actual attendance table - id of employee, and time scanned.
This is where things get complex - you'll need to calculate missed punches (scans), punch out and in for lunch, break, etc... Produce a daily hours report for each employee. Probably an exceptions report - ie: "Joe punched in at 8am, punched out at at 12pm for lunch, put out of work at 5pm" - so Joe missed his punch for returning from lunch. Policy will decide who enters that missed punch (now you need a web interface) and that it's documented. Also - employees can be shady when left un checked - what if another employee punches their friend in for them? Covering for them in other words because they might get in trouble for going over their half hour or hour lunch time?
How will the reports be viewed? Can the employee view the report? Now you need an auth system. Just management? Still some auth but not as complex. Can employees view their attendance log remotely or just while in the office? Can Management? Will management be ok with a web interface or will they want to download the report as a CSV? XLSX? PDF? Needs to be printable also so if there is an issue, HR can print the report and put it in the employee file.
Lots of moving pieces to this. You sound like you might be in a rush - this really isn't a weekend project, haha.