There are various different types of musical instruments. Which type do you like? I like percussion.
Please start by trying to play your instrument note for note.
"I forgot to bring your CD." "That’s okay. I can get it another time, but can you play an instrument for background music instead?"
Remember how 楽 was used in 音楽 and meant "music"? Well this is that 楽 again. A music container is something that can produce music. It is a musical instrument.
This is a jukugo word, which usually means on'yomi readings from the kanji.
You have already learned the reading for 器 but 楽 uses the secondary reading so you may not have learned it. However, you learned this reading in the word 音楽. Just bear in mind that the がく shortens to がっ.