I have an old MIDI keyboard and I would use it with FL Studio on the PC. How can I send notes of the keyboard to the computer making them recognize FL Studio?
You can buy a [u]USB MIDI Interface[/u] or a USB audio interface that also has a "regular old" MIDI port. You can get these from anyplace that sells musical instruments and related recording/PA equipment.
I wouldn't recommend building one yourself unless you REALLY want to learn all about designing & programming USB devices...
You'd have to make the USB device identify itself as a MIDI device and write the firmware to make it communicate with the proper MIDI class compliant Windows or Mac drivers with the proper protocol.
The good news is that the operating system has standard MID drivers so you don't have to write your own driver.
How can I send notes of the keyboard to the computer making them recognize FL Studio?
You can get a much cheaper MIDI → USB interface here:-
If you want to use an Arduino then you need to but or build a MIDI shield to convert the MIDI signals into serial data the Arduino can read. Something like this:-
Then you can use a helper app like Hairless in your PC to make this look like a MIDI device to your software. Alternately you can use a Teensy driver to make it look like a USB MIDI device. You can do the same thing if you choose the Arduino Micro or Leonardo as told here:-