In which sequence should I learn Chinese. What are simplified characters and important traditional characters which should I learn first and why those are necessary to learn first?
only if, you're interested in history, culture; or, you need to read materials dated before 1949; learn traditional chinese.
otherwise, learn simplified chinese first. particularly for anyone who learn "chinese" for bread-and-butter.
it's "easier" to read & write; for foreigners; compare to traditional chinese.
to be fair, well, there's a chinese proverb "物以罕為貴"; it's roughly "when a thing is scarce, it is precious".
have fun :)
I must say, the attitude towards learning Chinese is correct when you asked
Which should I learn first?
because you need to know both to be fully literate in Chinese. I'm also going to disagree with the other answer which suggests Traditional Chinese has a niche application and that Simplified Chinese is easier to read.
The most important factor is your environment!
If you're going to be moving to a Chinese-speaking area: Simplified Chinese has minimal usage in environments like Hong Kong and Macau and is practically not used at all in Taiwan and most established overseas Chinese communities. The areas that it is concentrated in are (1) Mainland China and (2) certain cities in Malaysia. Maybe you can add Singapore to that list, but their Chinese ability is mediocre at best, so English remains the most useful there.
If you're just studying as a foreign learner in your current location: Consider picking up the version which you would get most help from. If your friends or teachers surrounding you are mostly accustomed to Traditional Chinese, then you should go for Traditional Chinese. Ditto for Simplified.
If you're immersing yourself in Simplified Chinese materials, then pick Simplified Chinese. Ditto for Traditional Chinese, but the former is more likely - especially if you're planning to study in Mainland China or work as a translator.
It takes minimal effort to switch.
If you are fluent in one script, it takes very little time to switch to the other. Prioritise being fluent in one, rather than worrying about what you should choose first.
If somehow all other factors are equal, pick Traditional Chinese.
Simplified Chinese is not easier to learn than Traditional Chinese. See stats by IndexMundi*. Mainland China, the only fully Chinese region using Simplified Chinese, forms the lower bound of the literacy rate trends of the Chinese-using regions, indicating that script reform had minimal impact and economic development was the deciding factor by far.
Simplified Chinese instils a relatively poorer understanding of how the writing system and character components work. This is due to its overuse of writing abbreviations in its goal of cutting down strokes, meaning you'll be learning many components twice (an abbreviated version and a non-abbreviated version). Prepare for much more rote memorisation instead of exploiting reusable concepts if you choose Simplified Chinese.
In the wider cultural region of East Asia, Traditional Chinese is more useful. Historical China has influenced many countries in East Asia, including their vocabulary and writing systems. These influences were in the form of the Traditional Script, meaning you're killing multiple birds with one stone.
*Ultimately sourced from the CIA world factbook.