If I had to to narrow it down, it would be these four things.
1. Surround yourself with positive support
This is a big deal. You need to be around people who understand and support your efforts. Hanging out with people who tell you "haiya, won't kill you one la" as they hand you that can of coke for instance, will derail you and throw you off course. Positive vibes and energy will get you through tough times.
2. Discipline
Make a concerted effort to list down what you need to change and keep at it. To quote Kevin Kisner "This ain't no hobby fellas". Damn straight! It's not easy by any stretch and there will be days when you want to 'give up' or postpone your plan, which is why (1) is also very important.
3. There is no short-cut
This ties in with number 2. There really is no short cut. Don't hype yourself thinking you are going to lose 10kg in 1-2 months. It is unlikely and will end in frustration. Consistency is the name of the game and setting achievable targets is crucial. I.e. lose 1 - 2kg a month, exercise for half an hour at least 5 days a week.
4. Exercise and eating better
Exercise and diet go hand in hand.
Now when it comes to exercise, do what's comfortable for you and as long as you exercise and work up a sweat, you won't go wrong. I knew I couldn't sustain a heavy intense gym kind of workout long term, so I did my exercise on the treadmill (30-45mins sessions, 5-6 times a week) and gradually progressed from walking > brisk walking > to jogging. Sure I golfed on the weekend but with the golf buggy, I wouldn't exactly call it vigorous exercise.
Now when it came to food, I spent some time to really identify what I needed to cut out from my diet and what I could tweak and add to make it viable long term. I was pretty liberal with what I drank or ate previously and while I wouldn't call it reckless, I was eating a lot of food portion wise.
There are some who go on an extreme change and incorporate a "healthy" food diet, cutting out various elements completely from their diet. It may work for some but it really isn't for everyone. So what I did was cut out the unhealthy stuff (soft drinks), rarely ate processed foods, ate healthier (more veg, natural foods, ate more home cooked food) and controlled my food portions.
Controlling my food portions was somewhat a mental exercise. By controlling, I don't mean skipping meals. I was eating less-er and conditioning my mind to accept that I wouldn't die of starvation if I only ate 'this much food'. I am not kidding when I say I was a big eater. For example, lunch for me would be a serving of char kueh tiaw, topped with a fried egg and a side of 6 dumplings or nasi biryani with a side of roti telur or a Mcd large value set with an extra order of nuggets. You get the idea.
Everyone is different. As long as you cut out or eat less of the unhealthy stuff, tweak around with your diet and see what's best for you. By all means, have a cheat day once a week, eat that piece of fried chicken (home-cooked) but remember that balance and moderation are key.
I am no health guru, but the above really helped me to reach my health goals and maybe it will help you too.