Pick a progression type that matches what you want to play. Remember that your playing style can also affect the emotion of a chord progression.
Next, pick a key that you feel comfortable playing in. If you're playing guitar, the keys with the easiest chords are G major, E minor, C major and A minor.
Or why not just hit Randomize?
This section shows you all of the standard chords in the key you've chosen. If you want to put an extra chord in your progression to change it up, pick from one of these.
This is your chosen progression, in the key you selected. Sounding too happy? Try changing the key to minor. Too depressing? Change it to major.
This section does some magic with the circle of fifths to find some progressions that will probably sound good with the main progression.
The key might not match exactly in some of these, but you can use these to mix it up to put an interesting chorus or bridge in your song.