Learn Spanish (Latin America): Necesito tu ayuda.

Ti is a pronoun used when you want to say "you" after prepositions like a, de, or para (for example, para ti means "for you"). But here, we need the word "your", not "you". The word for "your" is the possessive adjective tu, so "your help" is tu ayuda.

Why doesn't tu have an accent mark here?

In Spanish, accent marks are sometimes used to tell apart words that sound exactly the same but have different meanings. Tu (without an accent) means "your", while tú (with an accent) means "you" as a subject (like in tú necesitas). Since we are saying "your help", we use tu without the accent.

I thought ayuda was a verb meaning "he/she helps". How is it used here?

Ayuda can be a verb (from the verb ayudar), but it is also a feminine noun that translates to "help" or "assistance". You can tell it's a noun in this sentence because it comes right after tu ("your").

Does tu change its ending because ayuda is feminine?