Lo chef prepara un piatto speciale.

Use il before most masculine singular nouns that start with a single consonant (e.g. il ragazzo, il libro). Use lo before masculine singular nouns that begin with:

• s+consonant (lo studente)
• z (lo zaino)
• ps (lo psicologo)
• pn (lo pneumatico)
• gn (lo gnomo)
• x, y (lo xenofobo, lo yogurt)
• other two-consonant clusters treated like the above (e.g. lo chef).

Italian has two masculine singular indefinite articles: un and uno.

• un goes before nouns starting with a vowel or a single consonant (un piatto, un cane, un’amica).
• uno goes only before nouns beginning with s+consonant, z, ps, pn, gn, x or y (uno studente, uno zio).
  Since piatto starts with a single p, you use un piatto.

Prepara is the third person singular (lui/lei/Lei) of the present indicative of preparare (“to prepare”). The subject is lo chef (a third-person singular noun), so you match it with prepara.
Conjugation snapshot:

• io preparo
• tu prepari
• lui/lei prepara
• noi prepariamo
• voi preparate
• loro preparano

Though chef is a French loanword ending in a consonant, it is treated as masculine by default in Italian. For a woman you have two common options:

• Say la chef (keeping the same noun but using the feminine article).
• Use the fully Italian feminine form la cuoca, which means “female cook.”

The noun chef is invariable in Italian, so the form stays chef. You only change the article:

• singular: lo chef
• plural: gli chef

1) Agreement: speciale is an -e adjective, so its singular form is the same for both genders.

• un piatto speciale (m. sg.)
• una torta speciale (f. sg.)
  In the plural it becomes speciali for both: piatti speciali, torte speciali.
  2) Word order: Most Italian adjectives come after the noun to describe an objective quality. You could place speciale before for a more poetic or subjective tone, but the neutral, everyday order is [noun] + [adjective].