The structure 首先…其次…
The structure 首先…其次… 
indicates that two actions happened one after another.
indicates that two actions happened one after another.Scheme
首先
action 1
其次
action 2
If a sentence has one subject, the subject is placed at the beginning of the sentence. Specifically, it is placed either before the structure or after 首先.
If a sentence has two subjects, the first subject is placed after 首先 and the second subject is placed after 其次.
Examples
不管做什么事情，首先你要明白你为什么要做，其次要知道怎么做
Whatever you do, you first have to understand why you are doing it. Second, you have to know how to do it
bùguǎn zuò shénme shìqing, shǒuxiān nǐ yào míngbai nǐ wèishéme yào zuò, qícì yào zhīdào zěnme zuò
首先放松下来，其次喝一杯茶吧
Calm down first and then have a cup of tea
shǒuxiān fàngsōng xiàlái, qícì hē yì bēi chá ba
This structure can also be used to indicate the degree of importance.
Examples
首先我是一个受害者，其次我才是一个警察
In this case, I was a victim first and a police officer second
shǒuxiān wǒ shì yí ge shòuhàizhě, qícì wǒ cái shì yí ge jǐngchá
它让你首先看别人怎样对待你，其次看你怎样对待别人
It first allows you to understand how others treat you, and then it allows you to understand how you treat others
tā ràng nǐ shǒuxiān kàn biérén zěnyàng duìdài nǐ, qícì kàn nǐ zěnyàng duìdài biérén