Learn Italian: Leggo un articolo su Roma. - I read an article about Rome.

Su does mean 'on' when talking about physical location (like a book on a table). However, it is also the standard Italian preposition used to mean 'about' when referring to the topic or subject of something, such as an article, a book, or a conversation.

Why isn't it sulla Roma? Doesn't su usually combine with the word for 'the'?

It would combine if the next word needed a definite article (like la). But in Italian, names of cities (like Roma) almost never take definite articles. Since there is no la for it to combine with, su remains a simple, uncombined preposition.

Where is the word for 'I' in this sentence?

It is built right into the verb! The verb leggere (to read) conjugated for 'I' is leggo. Because the -o ending already makes it completely clear who is doing the reading, Italians normally drop the pronoun io (I).

Why does leggo have a hard 'g' sound when the root verb leggere has a soft 'g' sound?

In Italian, the letter 'g' is soft (like in 'gem') when followed by 'e' or 'i', which is why leggere is soft. But when followed by 'a', 'o', or 'u', it becomes a hard sound (like in 'go'). Because leggo ends in 'o', the pronunciation naturally shifts to a hard 'g'.