A Closer look at が

This is the 'subject marker / particle'.

雨が降っています。　
ame ga futteimasu.
It's raining.

There is a subtile difference between WA and GA and I don't pretend to try to completely explain it. Years from now, you will still make WA/GA mistakes. Still, in general you can say WA is the main TOPIC and GA is the more specific SUBJECT at hand. In the above example we say it is raining. The topic isn't about rain. We are simply stating the circumstances at the moment and the subject of that particular sentence is rain. If we were to talk all about rain, we would probably start with WA as in:

雨は空から降ってくる水です。　
ame wa sora kara futte kuru mizu desu.
As for rain, it is water that falls from the sky. (You may go on to say more about the overall topic of rain.)

Used with SUKI

わたしは猫が好き。　
watashi wa neko ga suki.
I like cats.

Question words always use GA

何がおいしい？　
nani ga oishii?
What tastes good?

誰が来ました？　
dare ga kimashita?
Who came?

どこが一番いいところですか？　
doko ga ichi ban ii tokoro desu ka?
Where is the best place?