Connect a MIDI controller (like a MIDI keyboard) to your PC. A MIDI keyboard looks like a regular music keyboard, but allows you to input MIDI notes. If you have a USB controller, just connect it to your PC directly. If you have a non-USB controller, connect a MIDI cable to the "MIDI Out" jack on your controller, and then to your PC using a MIDI-to-USB interface (sold in music supply stores).
Open Ableton Live. Click the "Options" menu and select "Preferences." Click the "MIDI" tab on the left side of the pop-up window and select your MIDI controller from the options on the "Input" menu. Locate your device on the list in the lower half of the window and click the "Track" and "Remote" buttons so that they read "On." Exit your "Preferences" window.
Click the "Live Devices" button on the left side of your main project window. It is the second button from the top in the leftmost column of buttons. You will then see a list of instrument and effects folders. Click "Instruments" to access your VSTs and other software instruments.
Drag any desired VST instrument from the list directly onto the main project space (in the center of the window) to automatically create a new MIDI track. Your track will appear in two places: as a horizontal bar near the center of your screen, and as a track editor near the bottom of your screen. The track editor features various editing parameters (like Cutoff and Resonance), and you will use this small window to map your MIDI controller to your VST.
Record-enable your track so that you can access your MIDI and control your VST instrument. Just locate the button featuring a small round dot on the left side of your horizontal track view. Click it once to record-enable.
Click the "MIDI" button near the top of your project window to enter MIDI Map mode. A new window will appear, allowing you to map your connected MIDI controller to your selected VST instrument.
Click any parameter that you want to edit on the track editor at the bottom of your screen. For instance, you might click the "Volume" button if you want to assign a "Volume" setting to one of the buttons on your MIDI controller.
Move the knob or slider (on the controller) to which you want to map your selected parameter. You only need to move it slightly in order to send the necessary signal. Click the "Save" button at the top of your "MIDI Map" window. Repeat steps 7 and 8 for each additional parameter that you want to map between your controller and your MIDI instrument.