Learn Italian: La riunione di Anna è molto noiosa.

While most feminine nouns end in -a and masculine nouns in -o, some Italian words end in -e. These can be either masculine or feminine. You can tell riunione is feminine here because it uses the feminine article La and the feminine adjective noiosa.

How do we say "Anna's" in this sentence?

Italian doesn't have an equivalent to the English "'s" for possession. Instead, you always use the pattern "the [thing] of [person]". So, "Anna's meeting" becomes La riunione di Anna (the meeting of Anna).

Why doesn't molto change to molta to match the feminine noun?

Because here molto is an adverb meaning "very", and adverbs never change their endings. molto only changes its ending to match a noun when it means "a lot of" or "many" (for example, molta pasta).

Why does noiosa end in -a?

Adjectives must match the gender and number of the noun they describe. noiosa (boring) describes La riunione, which is feminine singular, so it takes the feminine singular ending -a.