First, to flip the stems just select the note or a group of notes and hit X.
Second, to select a group of notes, you need to hit shift, not Ctrl, although I think that's probably what you meant. Anyway, an easier way to select a bunch of notes is to simply select the first note of the group, then hold shift and select the last note of the group. All the notes in between will automatically get selected. For instance, if you have a string of of sixteenth notes on one staff that you want to select, then select the first note of the string, then hold shift and select the last note you want selected. Same with a chord. Select the bottom note of the chord, then hold shift and select the top note (or vice-versa), and with chords, this works on one staff or across both staves (or more depending on how many staves you have).
Hope that helped
Peace,
Bri