Warm ups are hugely important, especially as you get older.
A common concensus is a few minutes on a stationary cycle or rower, or the like, to raise heart-rate, then, as mentioned above by Faz and Tman, some light sets on the exercises you're working.
I also like to do some joint rotations and mobilisations, as my joints are getting old and knackered!
I also do some light rotator cuff exercises, as I've had a few years of issues with them in the past.
Stretching should only be done after the workout, as it can affect your performance - there was a study done on this years ago - and always once the muscle is warmed up anyway.
My warm up, as others do above, can take 20 minutes or so, in total. Just make sure it doesnt become a workout in itself....save your strength for the proper lifting.
Brix