Learn Portuguese (Portugal): Eu como todo o pão.

In Portuguese, when you want to say "all of the" or "the whole" of something, you usually include the definite article (o, a, os, or as) after todo. So, todo o pão means "all the bread" or "the whole bread". If you left out the o, it would change the meaning slightly to sound like "every bread".

Will todo change if I am talking about a different food?

Yes, it changes to match the gender (masculine or feminine) and amount (singular or plural) of the noun. Because pão is masculine and singular, we use todo o. If you were talking about a feminine word like sopa (soup), you would say toda a sopa.

I have seen the word como used to mean "how". Is this the same word?

They are spelled and pronounced exactly the same, but they are completely different words! Here, como is the "I" form (first-person singular) of the verb comer (to eat). The context of having the pronoun eu right next to it makes it clear that it is an action here, not the question word "how".

Since this lesson is about negatives, how would I say "I eat nothing" instead of all the bread?