Learn Italian: Il gatto è meno forte del cane. - The cat is less strong than the dog.

In Italian comparatives, 'than' is usually translated as di. However, because 'the dog' is il cane, you have to combine di with the definite article il. The words di + il always merge into the combined word del.

How would I change this to say the cat is 'more strong' (stronger) than the dog?

You would simply replace meno (less) with più (more), making it: Il gatto è più forte del cane. The rest of the grammar works exactly the same way.

Would forte change if we were talking about a female cat (la gatta)?

No, forte would stay exactly the same. Adjectives that end in -e in the singular are used for both masculine and feminine nouns. It only changes to forti if you are talking about more than one animal.

Why do we use articles here (Il gatto and del cane) instead of just saying Gatto è meno forte di cane?

Italian almost always requires definite articles (il, la, etc.) before nouns. Even if we are making a general statement where English might just say 'Cats are less strong than dogs', Italian keeps the articles.