Learn Italian: Ho molti ricordi della mia infanzia. - I have many memories of my childhood.

Even though the sentence is about childhood (a past time), the act of having the memories is happening right now in the present. If you wanted to say 'I used to have many memories' (implying you have forgotten them now), you would use the imperfetto: avevo molti ricordi.

Why is it della mia infanzia and not just di mia infanzia?

In Italian, you almost always need the definite article (the word for 'the') before a possessive adjective. So, you literally say 'of the my childhood'. Della is the combined form of di (of) and la (the). The main exception where you drop the article is with singular close family members, but infanzia is a regular noun.

Why does molti end in an -i here?

Here, the word for 'many' is describing the noun ricordi (memories). Because ricordi is masculine and plural, molto must change its ending to molti to match. If it were acting as an adverb meaning 'very' (like in 'very good'), it would stay as molto without changing.

What is the singular form of ricordi?

The singular is un ricordo (a memory). It comes directly from the verb ricordare (to remember), which you will use often when practicing the imperfetto tense.