が

が marks the subject: who does something, or what is or exists. Where は says "as for X", が points straight at X.

Three places a beginner meets it first:

• With あります and います: 猫がいます, "there's a cat".
• After question words, which never take は: 誰が来ましたか, "who came?" The answer keeps が: 田中さんが来ました.
• For something new you've just noticed: あ、雨が降っています, "oh, it's raining".

A rough test: if the sentence answers "who?" or "what?", the answer takes が. If it comments on a topic already on the table, は.

Likes, skills and understanding (好き, 上手, 分かる) also put the thing liked or understood before が, a point of its own later.