1. Getting a MIDI connection between your Yamaha P-155 and computer
You'll need a USB MIDI interface to connect the DIN MIDI ports on your Yamaha P-155 keyboard to a USB port on your computer.
More information: You can't use a USB A port to connect to a "computer host".
In general, USB A ports (flat rectangle shape) are on the end of a connection that acts as a "computer host", and USB B ports (square shape) are on the end of a connection that acts as a "peripheral device".
Yamaha keyboards typically have one or both of these kinds of USB ports on them:
a USB A port labeled "TO DEVICE" to connect USB thumb drives or other storage devices.
a USB B port labeled "TO HOST" to connect to a computer for USB MIDI communications.
Since your keyboard only has a USB A "TO DEVICE" port, you'll have to use your keyboard's DIN MIDI ports for MIDI communications. You'll need a USB MIDI interface to connect your keyboard to your computer, as shown in page 50 of the
manual. (Actually, page 50 of the manual is kind of confusing, it seems to say there are "two ways" to connect to a computer, but both ways described seem to be exactly the same thing -- use a USB MIDI interface to connect the DIN MIDI ports on the keyboard to a USB port on the computer.)
2. Record MIDI percussion sounds in GarageBand.
I'm a little unclear on how to tell GarageBand you want to record MIDI notes and have them sound like a drum kit, but I think you should try to create a "software instrument track" and select a "drum kit patch":
Intro to tracks in GarageBand on Mac
Intro to patches in GarageBand on Mac
You don't want to hear any sounds from your Yamaha P-155 keyboard as you play so you can either turn its volume all the way down, or set its Local Control mode off. (In the
manual, see "Basic Procedure in Function" on page 39 for how to select functions, then read the description of function "F7.3 Local Control ON/OFF" on page 44.)