Priorities

With food, the most important things to do in (approximate) order, no matter what your goals are:

1. Stop eating toxins. In rough order of importance from high to low: 
• junk food, processed food, and trans fats (usually these are a package deal)
• seed/vegetable oils
• sugar
• wheat and soy
• other grains and legumes
• dairy (for some people)
2. Sleep adequately. This means 8+ hours a day for most people.
• Most people will put this lower on their list or leave it off altogether, since sleep isn't directly linked to nutrition -- or is it? 
• Chronic sleep deprivation has a greater impact on long-term health, fat loss, and cancer prevention than any of the items that follow. 
3. Improve the quality of the food you do eat (switch to pastured meats, wild-caught fish, local/organic produce, add or increase intake of organ meats).
4. Supplement minimally and only as necessary (magnesium, vitamin D3, possibly zinc, selenium, vitamin K2, small doses of cod liver oil, etc.)
5. If and only if 1-4 are satisfied, then you can move on to tinkering with macronutrient ratios, macronutrient timing, intermittent fasting, more supplements, and so on. 

This can basically be distilled down to "Eat Real Food Only (and get lots of sleep)."

With exercise, the most important things to do in order, from the perspective of long-term health, longevity, and quality of life:

1. Stop being sedentary. Even the worst, most poorly designed exercise program is better than sitting on the couch watching TV. Daily walks are an excellent place to start.
2. Stop sitting all day. This is a corollary to #1. Basically, sitting all day is the equivalent of not sleeping enough. It seems harmless, but it is a huge cause of chronic pain, muscle loss, and fat gain. 
3. Rehabilitate any chronic injuries and resolve chronic pain through mobility/stability/posture training. 
• A good physical therapist is invaluable, but hard to find. In my experience, most PTs are a waste of time and money, and good PTs are rare. If you find a good PT, you should see him or her as often as you can afford it. 
• Daily mobility work is a must. MobilityWod.com is one of the best places to start for beginners who don't know what they are looking for (and it's free!). 
• If you have serious joint issues or chronic pain/injuries, and are serious about resolving them, I would highly recommend visiting a gym like IFAST or Cressey Performance and getting a personalized assessment and program. Do this even if you have to fly across the country to get there. 
4. Develop some basic strength. 
• Starting Strength alone will be better than 90% of fitness programs out there. (Yes, it's better than P90X and CrossFit.) Buy the book. It will be the best $30 you've ever spent on fitness. 
• Any reasonably healthy adult male under the age of 50 should, at minimum, be able to 
• back squat 1.5x his bodyweight, 
• deadlift 2x his bodyweight, and 
• do 5-10 dead-hang chin-ups, 10-20 push-ups -- all with perfect form.* 
• This should take around 1-6 months to develop for most people, with exceptions for morbid obesity and orthopedic issues. These will arguably be the most important 6 months of your training career, in terms of potential impact on long-term health and quality of life. If you can squat 1.5x your body weight at the age of 75, you are not going die in a nursing home. 
5. Do some conditioning ("cardio") that doesn't interfere with #3. 
• Short intervals of skipping rope, hill sprints, rowing, prowler/car pushing, etc. 
• CrossFit-type workouts can be useful here, but only if they are intelligently programmed and kept short (under 10 minutes, preferably under 7 minutes). The majority of random CF workouts that you'll find on the web do not fit this pattern, so do not go this route if you do not know what you are doing.
6. If and only if 1-5 have been satisfied, move on to more specific goals (sport-specific goals, competition, body composition**).

*These are conservative benchmarks. For adult women, I'm guessing:

• BS 1xBW, 
• DL 1.5xBW, 
• 1-3 DH chin-ups, 5-10 push-ups (not on knees). 

**For most people, body composition is their #1 priority when they sign up for a gym. As a trainer, my job is to figure out how I can accomplish this enough to make you keep coming back, without sacrificing too much of the more important items above (mobility, stability, strength). Luckily, strength training actually does a great job of improving body composition anyway, if you can get your client to just shut up and do it.