How do I know which key to press on my keyboard to get the kick drum or the cymbals for example ?
Pretty much all electronic instruments today follows the "General MIDI" standard which includes a definition for which notes should control which type of drum sound.
Note Drum Sound ==== ========== B0 Acoustic Bass Drum C1 Bass Drum 1 C#1 Side Stick D1 Acoustic Snare Eb1 Hand Clap E1 Electric Snare F1 Low Floor Tom F#1 Closed Hi Hat G1 High Floor Tom Ab1 Pedal Hi-Hat A1 Low Tom Bb1 Open Hi-Hat B1 Low-Mid Tom C2 Hi Mid Tom C#2 Crash Cymbal 1 D2 High Tom Eb2 Ride Cymbal 1 E2 Chinese Cymbal F2 Ride Bell F#2 Tambourine G2 Splash Cymbal Ab2 Cowbell A2 Crash Cymbal 2 Bb2 Vibraslap B2 Ride Cymbal 2 C3 Hi Bongo C#3 Low Bongo D3 Mute Hi Conga Eb3 Open Hi Conga E3 Low Conga F3 High Timbale F#3 Low Timbale G3 High Agogo Ab3 Low Agogo A3 Cabasa Bb3 Maracas B3 Short Whistle C4 Long Whistle C#4 Short Guiro D4 Long Guiro Eb4 Claves E4 Hi Wood Block F4 Low Wood Block F#4 Mute Cuica G4 Open Cuica Ab4 Mute Triangle A4 Open Triangle
In general, you can program your electronic drum kit and individual drum pads to assign any MIDI note that you want to any drum pad, and also to map or associate any drum sound that you want to any drum pad and to any MIDI note. It is up to you to make these assignments yourself.
If you want to know what MIDI notes and which sounds are assigned to which pads on your MIDI drum device by default when it comes from the factory, then the obvious answer is to read the owner's manual that came with your electronic drums. It will have a chart in the back listing all this.