Learn Arabic (Modern Standard): رأيت الممثلة التي حصلت على الميدالية.

التي (allatii) is the feminine form of the relative pronoun "who" or "which". We use it here because it refers back to الممثلة (the actress), which is a feminine word. If the sentence was about a male actor (الممثل), we would use الذي (alladhi).

Why is there على after حصلت? Does it mean "got on"?

In Arabic, the verb حصل (to get or obtain) is almost always followed by the preposition على (on). You have to learn them as a pair. Together, حصل على just means "got" or "obtained". So حصلت على means "she got".

Why does حصلت end with a ت?

The ـت (-at) at the end of حصلت (hasalat) makes it "she got" in the past tense. Even though the word "actress" is in the first part of the sentence, the verb inside the "who" clause still needs to match her gender.

Where is the Arabic word for "I" in "I saw"?

The "I" is built right into the verb رأيت (ra'aytu). The ـت (-tu) ending on a past-tense verb means "I did it", so you don't need to add the separate pronoun أنا (I) unless you want to add extra emphasis.

Is الميدالية related to the English word "medal"?