Learn Arabic (Modern Standard): ارفعي الوزن الثقيل يا صديقتي.

The verb ارفعي has the suffix ي (-i) at the end. This is the standard way to make an imperative (command) feminine singular in Arabic. If you were speaking to a man, you would drop the ي and just say ارفع.

Why do both الوزن (the weight) and الثقيل (heavy) have ال at the beginning?

In Arabic, adjectives must perfectly match the noun they describe. Because the noun الوزن is definite (it has the word for "the" attached), the adjective الثقيل must also take ال to show they go together as "the heavy weight".

Why is the word يا used before "my friend"?

The word يا is a calling particle used whenever you address someone directly. While we usually leave it out in English translations, in Arabic you almost always use it when talking to someone. Saying just صديقتي on its own here would sound unnatural.

What happens to the ending of the word for "female friend" (صديقة) when we add "my"?

The word صديقة ends with a taa marbuta (the letter used for feminine nouns). When you attach a pronoun suffix like the ي for "my", the taa marbuta "opens up" into a regular letter taa. That is why it is spelled and pronounced صديقتي (sadiqati).