"How do I start?" is a common question people ask when considering improving their diet and making healthier choices.
Whether for a specific period of time or on an ongoing basis, the best way to start is to prepare your environment (pantry, fridge and freezer) and plan what you're going to be eating then batch cook into individual portions – also known as meal prepping.
You can prep for up to a week (and longer if your meals go straight into the freezer for later), depending on how much space you have, how many people you're prepping for, what kind of food you're making (eg. vegetable dishes will last better and longer than meat-based ones) and your schedule.
Stress: You always know you'll have a healthy meal and won't feel tempted to buy something that's not.
Go through your pantry, fridge and freezer and bin the opened or out-of-date items, donate any canned goods to charity, or even give leftovers to friends. Just get rid of all the junk. If there's anything your family will eat that you'd prefer not to, move it to another shelf where you can't see it. (Or just try to get rid of it without them noticing!) And a word of warning: do NOT keep that unopened packet of chocolate biscuits – you will eat them in a moment of weakness.
Existing food: What you already have in the pantry/fridge/freezer that you can use, and what you'll need to buy.
Once you've chosen your recipes, make a list. It's easy if you divide everything into sections of where you need to go to get it: e.g. supermarket, farmers' market, butcher, health store. Make it comprehensive and only stick to items on the list (no extra potato chips if you're hungry!).
Bulk whole foods stores are the best place to get pantry items – think coconut flakes for $3/kg (instead of $30/kg packaged from health food stores), nuts at half the price of chain supermarkets, hard-to-get spices and a wide range of local and organic products.
After you're done shopping, wash and store everything properly. My post How to Wash and Store Your Fresh Produce has good tips.
I hope that helps! Let me know if you have any good tips yourself.