Dialugues　

1. 请问，邮局在哪儿？ Qǐngwèn, yóujú zài nǎr?
Please may I ask, where the post office is?
post office, 邮局 yóujú.
邮 yóu, post.局jú is a subdivision of a governmental department.
So if you wanted to go to a post office, you could just say 邮局在哪儿? yóujú zài nǎr? Where is the postoffice?
Post office is 邮局 yóujú. So “Where is the post office?” is 邮局在哪儿? yóujú zài nǎr?
In order to sound polite, you can say 请问 qǐngwèn, please may I ask, at the first of the sentence.
请问，邮局在哪儿？ Please may I ask, where the post office is?
 

2. 我要寄包裹。 Wǒ yào jì bāoguǒ.
I'd like to send a package.
If you want to send a package, you can say 我要寄包裹.
我 wǒ, I.
要 yào, want to.
寄 jì, mail.
Package in Chinese is 包裹 bāoguǒ.

3. 多长时间能到？ Duō cháng shíjiān néng dào?
How long will it take to arrive?
多长时间 duō cháng shíjiān, how long.
能 néng, could,
到 dào, arrive.

4. 我要取包裹。 Wǒ yào qǔ bāoguǒ.
I'd like to pick up a package.
我要 wǒ yào, I’d like to do something.
取 qǔ, to take.
And package is 包裹 bāoguǒ.

5. 我要买几张明信片寄给朋友。 Wǒ yào mǎi jǐ zhāng míngxìnpiàn jì gěi péngyou.
I want to buy some post cards and send them to my friends.
我要 wǒ yào, I want to do.
买 mǎi, to buy.
几张 jǐ zhāng, several pieces of. 张 zhāng is a measure word for paper, painting, tickets, etc.
明信片 míngxìnpiàn, postcard.
寄给 jì gěi, to send to.
朋友 péngyou, friends.