Another important concept is progressive overload. Start with weights where you can do at least 6 reps comfortably (preferably
. When you can do 12, you're ready to up the weight by 5-10% or so.
Make sure you work opposing muscle groups equal reps. That is, if you do chest press, offset this with bent-over rows; squats with deadlifts, biceps curls with triceps extensions, etc.
Work biggest muscle groups first and foremost: legs, chest, shoulders, back. Have in mind that compound exercises will work out secondary muscles, too, so for example you probably don't need to worry about arms a lot from the get go, since they get enough work from chest and back exercises.
Several things that are my personal preference, and probably not everyone will agree:
Work more free weights, less machines -- you get more freedom of motion with these.
I don't do squats, deadlifts, or anything that includes bars over the head. Yes, I know squats are supposedly the most efficient lower body exercise, but I value the health of my back and knees too much for this
These could easily be replaced with less taxing exercises (leg press, pull-ups, etc.)
Finally, understand that even with a bad workout regimen you'll get eventually get gains -- the only difference is how quick you'll get them. And when you're starting from scratch, gains are usually easier.
Also, when I was getting back into weights a couple of years ago, I found this website very useful in terms of tips for setting up a routine:
http://www.aworkoutroutine.com/the-ultimate-weight-training-workout-routine/