Learn Italian: Oggi vado a prendere mio figlio a scuola. - Today I am going to pick up my son from school.

When a verb of motion like andare (to go) is followed by an infinitive verb, you must connect them with the preposition a. So vado a prendere means "I am going to pick up" or "I go and pick up".

Doesn't prendere normally mean "to take" or "to get"?

Yes, prendere is a very versatile verb! In the context of people or things waiting at a location, the phrase andare a prendere is the standard, idiomatic way to say "to go pick [someone/something] up".

Why is it just mio figlio instead of il mio figlio? I thought possessives needed definite articles.

You are right that possessives usually take a definite article (like la mia macchina), but there is a major exception. For singular, unmodified family members, the article is dropped. Because figlio is a singular family word, we just say mio figlio.

Why does the English translation say "from school" when the Italian preposition a usually means "to" or "at"?

In Italian, the location where you are picking someone up is treated as the place where the action happens, so it takes the preposition a (at). Literally, the sentence says you are going to pick up your son "at school", which we naturally translate to English as "from school".