How can I play the drums with my computer keyboard?
However, some DAWs offer a native "virtual MIDI keyboard" which can be played with the mouse or computer keyboard while the window is open.
In Reaper you can bring it up by going to:
View -> Virtual MIDI Keyboard
In Cubase you can bring it up by going to:
Studio -> On-Screen Keyboard
If the virtual keyboard of your DAW doesn't suffice you and you are looking for something more customizable in size and mapping, download and install the following tools and afterwards follow the video.
Download LoopBe1 virtual MIDI device
(scroll down on the page)
Download Virtual MIDI Keyboard software
The video shows how to set these tools up once they are installed. In the example Reaper is used as a DAW, but it works similar in other DAWs, once you set "LoopBe Internal MIDI" as a MIDI input device for your MIDI track.