Plants should make up the vast majority of your diet. You don’t need to cut animal foods out of your diet entirely, but keeping them to a minimal portion is best. Eating only plant foods one day a week has a positive effect, so think what effect 5 days of vegetarianism a week can have.
Most of the healthiest cultures around the world eat mostly plants. Some of them are vegetarians, and many of them occasionally eat meat. Some of them don’t eat meat like we think of meat, but they may eat a small amount of insects (don’t worry, I won’t make you do that). But the focus is on plants, mostly fresh and in season.
You can get pretty much all of your nutrition from plants, plus a little bit of supplementation.