For your first "get back to the gym week", I would suggest:
Pump yourself up mentally before going in...listen to some upbeat music that makes you happy. Have a glass of water or milk with a scoop of peanut butter and get out of the house!
Get in and get right to the bikes or ellipticals for about 6 minutes.
Do some light upper-body warm-up work, like hanging stretches alternating with push-ups.
Work the larger body parts, then move to smaller muscles like arms, delts, calves, etc. Do only a couple of sets of everything.
Finish up with some sets of upper and lower abs, followed by full body stretches, like hanging from a pull-up bar or using a stretch cage (if your gym has one).
You'll feel tremendously energized but not beat up....and ready to get into a routine and a well-thought-out plan such as what others here have suggested.