If you're interested in creating songs or remixing existing ones, you need a way to sample existing songs. Sampling, or also called splicing, is the method of grabbing segments from audio to be used in mixes and more. With these audio clips, you set them to specific pads on your MIDI hardware. Now there is professional software out there that can do this, but I prefer a free method using software I already have installed unless I really need the advanced features.
Using Audacity, you can simply turn one song into many segments and clips and have them export all at once. Or, if you have a LP recorded you can split it into the proper segments. You will need to download Audacity, which is free and very fast to install. Other than splicing music, Audacity can do much more, such as recording audio and performing some basic effects to existing audio.
Importing Audio to Audacity
To begin, you need to open your audio file in Audacity. Some songs will be protected and audacity won't allow you to edit them and sometimes won't allow you to open them. You will need files that you properly own without protection. Also, when re-using other people's work, you need to have their permission unless this is used only for personal use (not releasing it or using it anywhere).
Drag your audio file right into Audacity (anywhere). You should see a new audio track appear. You can play it by pressing the Play Arrow, or by pressing space on your keyboard.
Creating a Label / Splice
Move your cursor and click to a part of the song you want to have a segment of. You can zoom in using ctrl + mouse wheel. Once your marker is in the correct location (a line appears where the marker is), you can add a new label.