を indicates the subject of the action. For example, 本を読む。Ich lese ein Buch, 球を投げる。Ich werfe einen Ball. It also indicates the subject of the action in causative sentences. See this one, 子供を泣かせる。Ich mache ein Kind weint. を denotes the place where one's action is taken at. 山道を登る。Ich Aufstieg Berg Straße. It denotes the starting/seperating point for the action. 車をおりる。Ich mich aus einem Auto, 席を立つ。Ich lasse meine Sitz, 毎朝8時に家を出る。Jeden Morgen ich lasse meine Heimat um 8.00. を denotes the period that the action lasts. 一日を日本語勉強で過ごす。Ich verbringe einen Tag zu lernen Japanisch, 一夏を海辺で暮らす。Ich verbringe einen Sommer am Strand. Lastly, を denotes the subject that your likes and dislikes or wishes are pointed at. For instance, 水を飲みたい。Ich möchte Wasser trinken. However, が is sometimes used INSTEAD OF を when some verbs like 好きだ(mögen), 嫌いだ(hassen)、上手だ(etw. gut können)、下手だ(etw. schwach sein)、できる come. 何々がほしい。Ich möchte was. 君が好き。Ich mag dich. 日本語が上手だ。Ich bin fließend in Englisch. 虫がきらい。Ich hasse Insekten. As for が, it indicates the subject of the action or condition or nature. 鳥が鳴く。Ein Vogel weint. 雪ゆきが降ふる。Es schneit. 花が美しい。Die Blume ist schön. 負まけるが勝かち。Verlieren ist wie gewinnen. が indicates the subject that your likes and dislikes or wishes are pointed at, or that you are able to do. 英語が出来る。Ich kann Englisch sprechen. 時間かねがほしい。Ich brauche Zeit. 映画が好きだ。Ich mag Filme. 母がこいしい。 Ich vermisse meine Mutter.

it depends on a verb follow by. Basically, you have to use "wo" in SVO sentences like I wrote a letter. watashi wa tegami wo kaita. I played the piano. watashi wa piano wo hiita. but there are a few exceptions replacing "wo" to "ga" about some verb. I like cat. watashi wa neko ga suki. I hate dog watashi wa inu ga kirai. "suki" and "kirai" are verbs followed by "ga".

Ga is used for adjectives while Wo is used for verbs. example: pizza wo tabemashita. I ate pizza watashi wa umi ga suki jaa arimasen deshita. as for the sea, I don't like it.

wo, を, is the direct object particle. It's used with transitive verbs to determine to what/who the action is being done to. 手紙を書きました。Tegami wo kakimashita. I wrote a letter. ga, が, doesn't have anything to do with verbs really. See http://nihongo.3yen.com/2005-02-05/the-difference-between-and/ for its usage. It can only really be confused with ha, は.