Launch Ableton and open a project. Click on a MIDI track within the project.
Click the "Live Devices" icon at the far left of the program window. Click the arrow next to "MIDI Effects" to expand it. Click the arrow next to "Scale" to open the "Scales" section. Ableton includes a variety of built-in key signatures, including minor, major, pentatonic and whole tone.
Click a key signature -- "C Major," for example -- and drag it onto the MIDI track. Click and drag the "Base" knob to choose a key signature.
Start Ableton and open a project.
Click the first number in the "Time Signature" box in the top-left corner of the Ableton window. Type in a new value; for example, if you're planning to compose music in 3/4 time, type in "3."
Click the second number in the "Time Signature" box. Type in a new value. Click anywhere outside the "Time Signature" box to set the new time signature.