The way I look at it is that your current thoughts and habits have produced your current size. If you want to be a different size, you start by identifying thoughts and habits that aren't serving you, and adding new ones that do. I throw thoughts into the equation because if you're operating from a place of shame, guilt, fear, restriction, or desperation, that's going to undermine any new habits and promote reactive overeating. The way you view the process really matters. If you're calm, curious, owning your choices, and eating to feel good, wow! Very different experience.
I also like to think in terms of values instead of goals. I value health and leanness, but I also value fun and flexibility, so that keeps me from doing anything miserable. One of the biggest causes of internal conflict and drama is behaving in ways that conflict with your values. If you value freedom and spontaneity but you lock yourself into a rigid calorie counting meal plan, it's not going to end well. If you value feeling lean and fit but you're not exercising at all and you're overeating fast food every day, that will feel pretty awful.
Another important concept is eating to feel good. You deserve to feel good right now. You deserve to enjoy your food and your life. This seems like crazy talk for those of us who have been driven by "shoulds" and external rules, but it's the key to sustainable habits. You won't keep doing something that doesn't feel good. You'll look for an escape. The idea is to create a positive feedback loop where the new behaviors are rewarding enough that you want to keep doing them. That usually means going for simple changes with an immediate payoff, like eating more fresh fruit, which is easy and tastes good. Or eating premium ice cream a couple of times a week instead of overeating fat-free frozen yogurt every night. It tastes better and results are better, so win/win. An example of a wrench in the positive feedback loop is something like setting an alarm to get up and run every day at dawn if you hate running. Maybe you'll lose a pound or two at first but it won't be sustainable because it's not rewarding.
Think about your habits. Which ones are working for you and which ones aren't? Consider your values. What's important to you? Can you see instances where your behaviors are at odds with your values?
That's how you start, by getting a clear picture of what's happening now, then looking for ways to adjust. Think in terms of the big picture and being consistent over time.