Be careful for what you eat or Be careful what you eat

Be careful what you eat is the normal expression, with Be careful of what you eat a possible alternative. I can't imagine any native speaker saying Be careful for what you eat.

Here's a relevant Ngram

Be careful for is rather archaic. You may have heard it from Biblical quotations, for example Philippians 4:6 KJV, "Be careful for nothing" where it means "Do not be anxious about anything". From the Ngram you can see "Be careful for" has never been very widely used, and the specific phrase "Be careful for nothing" accounts for around half of all occurrences.

I suspect that you do not mean "Be anxious about what you eat," anyway; merely "Pay attention to it." [Edit: I doubt that Eminem meant "Be anxious" either, but poetry/music (or rap) is different from normal prose or speech.]

Normally these days, be careful is paired with about or of, and I believe those prepositions have subtly different usages which are principally idiomatic and difficult to formalise. My experience matches the Ngram and about is used more often than of. You are unlikely to go wrong using about.