Vegetarian/vegan

Maybe if you told us what part meat and dairy play in your diet currently or the recent past, we could offer more specific advice. I.e., what/where are you trying to make substitutions?

If you really want to start from a blank slate, the difference between being an omnivore and being a dairy-free vegetarian is that you need to get your protein from non-meat, non-dairy sources. So, you need to eat legumes (beans, peas, lentils) and legume-based foods (soy milk, tofu, edamame, hummus, some tempeh), seitan (made from the wheat protein gluten), eggs if you eat eggs, nuts and seeds (but those contain a lot of fat, which means a lot of calories, so portion control is important), grains, TVP (texturized vegetable protein) and TVP-based faux-meat products, and quorn (made from a fungus, like mushrooms). Then there things that I think of more as supplements, like vegan protein powders and nutritional yeast. Try to incorporate one or more of these foods in most meals and snacks.

If you're not eating eggs, you should look for B12-fortified foods or take supplements.

What's your motivation to stop eating meat/dairy? (Ethical, specific health issue, etc.)

Just generally: Try to move past the "one big protein per meal" idea that's the meat-eaters' common approach. ("What's for dinner?" "Chicken" "Steak" etc.).

Your larger protein sources are still important, but think about also getting small additional amounts from other foods you eat, too. (Some of the meat substitutes or alternatives are more calorie-dense than lean meats or seafood or nonfat dairy, so getting a bit of protein from lots of foods is a useful strategy. There are veggies with protein, fruits with protein, breads/grains with more protein than others, etc.).

I've been vegetarian (though ovo-lacto) for 45 years now. I personally don't enjoy fake meat or protein powder/bars, so I pretty much never eat those (life is way too short to eat things I don't consider tasty). However, if you do like those things, they can be a help for getting well-rounded nutrition. (If anyone tells you that vegan/vegetarians need less protein than meat-eaters do, find a better source of nutritional information. Happily, it's very manageable to get sufficient protein and generally good nutrition from plants.

Keep an eye on iron, calcium, and B12. If vegan, you'll need to supplement B12 somehow; nutritional yeast that's enriched with B12 can be a good source.)

If you want to be strict about avoiding any animal foods, watch out for gelatin and things with broth - these are common stealth sources.