The idea behind this usage of 要 (yào) is that someone is "planning to" or "going to" or "preparing to" do something. It's not that they just want to, or have to, it's that they fully expect to do it. It's in their plan.
This usage of 要 (yào) typically includes a time word of some sort, which may be placed before or after the subject. This is fairly logical; if you're discussing plans for the future, you're quite likely to say when you plan to do things.
Subj. + Time + 要 + Verb
Time + Subj. + 要 + Verb
Note that the time word is not strictly required, and when it's missing, you may at first find yourself wondering which usage of 要 (yào) you're dealing with. This is normal; it just takes some getting used to.
- 我 明天 要 买 一 个 iPhone。Wǒ míngtiān yào mǎi yī gè iPhone.I am going to buy an iPhone tomorrow.
- 你们 现在 要 出去 吗？Nǐmen xiànzài yào chūqù ma?Are you all going out now?
- 我们 今年 要 去 美国 。Wǒmen jīnnián yào qù Měiguó.We are planning to go to the U.S. this year.
- 他 下 个 月 要 来 中国 工作。Tā xià gè yuè yào lái Zhōngguó gōngzuò.He is coming to China to work next month.
- 你 下 个 星期 要 去 她 家 吃 晚饭 吗？Nǐ xià gè xīngqī yào qù tā jiā chī wǎnfàn ma?Are you going to her place for dinner next week?
- 这 个 星期天 你 要 做 什么？Zhège Xīngqītiān nǐ yào zuò shénme?What are you doing this Sunday?
- 我 晚上 要 给 妈妈 打电话。Wǒ wǎnshang yào gěi māma dǎ diànhuà.I am going to call my mom this evening.
- 下午 老师 要 来 我家。Xiàwǔ lǎoshī yào lái wǒ jiā.The teacher is coming to my place this afternoon.
- 老板 明天 要 见 他们 吗？Lǎobǎn míngtiān yào jiàn tāmen ma?Is the boss going to meet them tomorrow?
- 下班 以后 你 要 回家 吗？Xiàbān yǐhòu nǐ yào huíjiā ma?Are you planning to go home after getting off work?