After years of unstructured lifting you probably know how to do the lifts, but your routines are probably just a collection of exercises. You probably know how to do all sorts of lifts, but not how to put them together to make progress. Now is your chance to learn about programming.
Do Madcow for like 6-8 weeks. Get used to the higher frequency, different numbers of worksets on different days, and the heavy/light/medium setup. See how your body responds to the basic, full body approach. After 2 months, you should kind of "figure the program out." You'll understand how its supposed to work, and you'll find how it works for you and what areas it improves or doesn't improve. Also, if you've never done a set routine before you'll probably be pretty bored with it by then.
After that, try a basic body builder type routine for 6-8 weeks. Here's were you learn about assistance exercises and higher rep ranges. After the Madcow program you'll probably be anxious to do some dumbbell flies and tricep extensions. Get a little swole, and see the differences between training for a pump and training the basics for strength.
When you get bored 6-8 weeks later try 5-3-1 for two months. This will teach you about wave loading, deloads and periodization.
After that you should be bigger, stronger and wiser.