chigai_wa-ga [EN]

が is used:

１. When describing a topic introduced for the first time or when you particularly want to emphasize something.
　昔、ある村に太郎という若者が住んでいた
　Long ago, in a town, there was a young person named Tarou
　明日、首相がこの市に来る
　Tomorrow, the prime minister will come to this city

２. In describing natural phenomena and things you have seen or heard in person

　久しぶりに公園を散歩した。桜がとても美しかった
　After a long time I took a walk in the park. The sakura were very beautiful.
　ここはいい所だ。さわやかな風が吹いて、木の上で鳥が鳴いている。
　This is a good place. A fresh wind is blowing, and the birds from the trees are singing.

３. When highlighting the subject in statements in which the noun is modified, or which express conditions, time or reasons.

　あの作家が書いた本を一度読んでみるべきだ
　You should read at least once, the books of this author.
　ヤンさんが来たら、この書類を見せよう
　When YAN san come, I'll show him/her the documents.

は is used

１. When raising a topic, when referring back to a topic mentioned before.

　日本語能力試験は大勢の人が受験する
　A great number of people take the JLPT.
　昔、ある村に太郎という若者が住んでいた。太郎はとても貧乏だった。
　Long ago, in a town, there was a young person named Tarou. Tarou was very poor.

２. When drawing attention to, emphasizing or negating a topic.

　日本では車は道の右側を通ることになっています
　In japan, the cars run through right side.
　あの人は結婚しようとは思わない
　That person does not think about marriage.

３. To explicitly contrast two things

　梅の花は咲いていますが、桜はまだです。
　The flowers of the japanese plum are blooming, while the flowers of the sakura have not yet.
　雪は降っているが、風は吹かない。
　Is snowing, but there is not wind blowing.
　外ではコートを着ますが、部屋の中では脱ぎます
　I use a coat outside, inside my room I take off it.

Source: Shinkanzen Master N3 Bunpou Page 136 and 137