"I have a fish": "ho un pesce"; or: "io ho un pesce" - in Italian you usually don't need the personal pronoun as a subject, because you can already understand from the verbal form if it is a first, second or third person, singular or plural. Unless you want to stress it, for example if you say "io ho un pesce, loro no!" ("I have a fish, they don't!").
The articles la and il correspond to the, and you always need to have either a feminine or a masculine form. The same for un (uno), una, which corresponds to "a". But, in plural: "io ho dei libri" ("I have some books"). "Dei, delle" is a partitive, and you use it as the plural equivalent of "un, una". A "bad" thing about grammatical gender: every noun is either masculine or feminine, also "il tavolo" (the table) or "la luna" (the moon), and one just needs to learn every word with its article - although of course we would understand what you want to tell also if you say "il luna" - but it would sound strange. Luckily, you can usually (but NOT always) understand whether a noun is feminine or masculine from its ending, fo example -a feminine, -o masculine.