Want to transition to mainly vegetarian diet

I've been vegetarian (ovo-lacto) for 45+ years.

My best advice for getting enough protein is to break out of thinking "one big protein as the center of each meal", as meat-eaters commonly do ("What's for lunch?" "Burger" "What's for dinner?" "Chicken" ).

It's still a good plan to get protein-dense foods at each meal, of course (beans and other legumes; seitan if you don't have gluten issues; soy foods like tofu or tempeh; etc.). But a good adjunct is to think in terms of leaning toward getting a bit of protein from many other foods you eat: There are grains or grain-equivalents with relatively more protein (quinoa is one), types of pasta with more protein (chickpea, soy, black bean), veggies with more protein (broccoli, etc.), seasonings with a bit of protein (miso, nutritional yeast), even fruits with some protein (guava). Prefer breads with more protein to those with none, crackers with more, snacks with more (like crispy chickpeas or broad beans, soy nuts) etc.

Those extra bits of protein through the day really add up, and getting protein from a wide variety of sources helps with balancing essential amino acids (EAAs). We don't need strict EAA balance at each meal, as was once thought, but over relatively short time horizons the variety will help. Consider vegetarian protein combinations from cultures with strong vegetarian traditions, such as beans and rice: They're often more balanced in themselves.

Those cultures are also a good source of recipe ideas: Indian cooking, in particular, has a really rich vegetarian tradition, as do some of the Asian countries, some indigenous cuisines of South America and Africa, etc. Good stuff!

Many people enjoy the meat substitutes that are so widely available now, the meat-mimics like Beyond or Impossible, or some less-close faux meats like Quorn, Gardein, etc. I personally don't enjoy them (purely a matter of taste or satiation for me, nothing wrong with them in the abstract, especially if you enjoy meaty flavors), so I don't eat them (ditto for protein bars or powders: they're fine, but I don't love them). I'm mentioning them partly because there's a trap that some new vegetarian eaters fall into, of assuming the nutrient profile of a meat-substitute is similar to the meat it replaces. In some cases, that's true. In others, it's dramatically untrue. For example, green jackfruit is a common substitute for pulled pork, and it's pretty tasty in a BBQ-style sandwich, but it has really negligible protein.

Also, you'll find a few vegetarian or vegan advocacy sites that suggest people need much less protein than mainstream recommendations suggest. There really, really is not good evidence for this. In fact, it may be good to err on the side of getting a little extra, if one wants to be conservative about that EAA balance issue.

BTW, I hope you're not thinking that vegetarianism is a route to weight loss or ideal nutrition: Sadly, it isn't necessarily so. Most people (veg or omni) would be well served by eating plenty of varied, colorful veggies and fruits, well beyond population-average intakes. Those foods can be quite filling for some, so potentially a useful factor in weight management. But I'm living proof that it's possible to become overweight and obese while vegetarian (and also to lose weight eating the same way).

Best wishes!