Learn Polish: Zamawiam gorącą pizzę. - I am ordering a hot pizza.

Pizza is the direct object of this sentence — it is the thing being ordered. In Polish, when a feminine noun ending in -a becomes a direct object, its ending changes to -ę. This is called the accusative case.

Why does the adjective gorącą end in -ą instead of -ę like the noun?

Adjectives and nouns have different rules for their endings. For feminine direct objects, adjectives change their -a ending to -ą, while nouns change to -ę. That is why we say gorącą pizzę and not gorącę pizzę.

Why is there no word for I in this sentence?

It is built right into the verb! Zamawiam means I order or I am ordering. Because the -m ending already tells us exactly who is doing the action, Polish usually drops the pronoun ja (I).

Does zamawiam mean I order or I am ordering?

It means both! Polish does not have a separate continuous tense like English does. Depending on the context, zamawiam can mean that you order something regularly, or that you are ordering it right now.