What is the best advice for someone who is a vegetarian is to?

My best advice is to do research before you starting removing meat from your diet. I know several people who accidentally got sick because they tried to live off of potato chips and celery. Research different meal idea, recipes, meat replacements, and benefits of vegetarianism.

Next, decide what's in/out.

Ovo-vegetarian: eats eggs but meat, fish, dairy, poultry is out

Lacto-vegetarian: eats dairy products but meat, fish, eggs, and poultry is out

Lacto-ovo-vegetarian: eats eggs and dairy products but no meat, fish, or poultry

Pesce-vegetarian: eats fish, eggs, and dairy but not meat or poultry

Good luck!