Eggs, chicken, salmon, tuna, tofu, full-fat Greek yoghurt, lentils, beans of various kinds are great sources of protein. Many people find they need fewer calories as they get older, so it's best to make sure the calories one eats are nutritious. I don't have breakfast, so I typically eat variations of the following:
Brunch: 2 egg omelet plus salad, or salad with beans/chickpeas/lentils or leftover salmon/chicken or a can of tuna
Snack: fruit, a handful of nuts, a boiled egg or some yoghurt with berries (frozen is fine)
Dinner: fill 2/3 of your plate with vegetables and/or salad, the rest protein: chicken, fish, tofu etc.
I panfry/braise/roast chicken, fish or tofu in batches and eat them over 2-3 days. I always have boiled eggs in the fridge - they are known as protein bombs.
Maybe do a bit of reading, e.g. The Sugar Solution or Ultraprocessed People. Dr Becky Gillaspy on YouTube has lots of bite sized talks that are interesting and informative.
Lucy Wyndham Read is good if you are unfit or not used to exercise. If you are more energetic, try Growingannanas. If you are serious and willing to work out with dumbbells (highly recommended!!), aim for Caroline Girvan. All free on YouTube.