The difference between 은/는and 을/를

은/는 is a topic marking particle. This means that it is used to designate the main idea or topic of a sentence. 은/는 is added to the end of what the speaker wants to talk about or explain in a sentence.  You could roughly translate it to mean “as for” or “regarding” in English.
Example:

• 저는 미국 사람입니다. (Jeo-neun miguk saramimnida): As for me, I’m American.
• 저 직업은 의사입니다. (Jeo jikeop-eun ouisaimnida): Regarding my job, I’m a doctor.

은/는 can also be used to refer to something mentioned earlier or when talking about something already known by both sides of the conversation.
Example:

• 저는 네일 요코 씨를 만나요. 요코 씨는 일본에서 왔어요.(Jeo-neun naeil Yoko ssi-reul mannayo. Yoko ssi-neun ilboneseo wasseoyo.): As for me, tomorrow I will meet Yoko. Yoko came from Japan (Since Yoko was talked about in the previous sentence, 는 is added).

은/는 can also be used to compare or contrast two things by adding 는/은 to what you want to contrast (ie. subject of the sentence, object of the sentence, etc.)
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을/를 is used to mark the object of the sentence. It is added to nouns, and many verbs (such as 보다, 먹다, 읽다, 마나다, etc.) need this object particle.
Examples:

• 영화를 봐요. (Younghwa-reul bwayo): I watch the movie. (The movie is the object of the sentence, so 를 is added.)
• 한국어를 배워요.(Hangukeo-reul baewoyo.): I study Korean. (Korean is the object of the sentence, so 를 is added.)

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To summarize, 은/는 marks the topic of the sentence, while 를/을 marks the object of the sentence. Hope this helps!^^