The particle が is used to indicate the subject of a sentence. It can point out who or what is doing an action, or it can specify something that possesses a particular quality, often presenting this information as new or important to the listener.

• 猫(ねこ)が寝(ね)る。
  The cat is sleeping. (The cat is the one that is sleeping)
• 犬(いぬ)は鼻(はな)が長(なが)い。
  The dog has a long nose. (The dog's nose is the thing that is long)

In these examples, が places emphasis on the subject itself, singling it out as the one performing the action or having a certain characteristic. For this reason, it's often said that が spotlights the word preceding it, whereas は draws attention to the information that follows it.

• 音楽(おんがく)が好(す)きです。
  I like music. (Music is the specific thing that is liked)
• 宿題(しゅくだい)は嫌(きら)いです。
  I dislike homework. (Speaking of homework, I dislike it)

The particle が is used to identify a specific subject when other alternatives are possible. This makes it ideal for describing events that you are observing directly or for pointing out something as a unique occurrence.

• 桜(さくら)が咲(さ)く。
  The cherry blossoms are blooming. (As opposed to other flowers or trees)

Since the subject of this sentence is just one specific thing among many other possibilities that could be discussed, が is the appropriate particle.