There are many sects in Islam. Which one should be followed?

Misconceptions About Islam - By Dr. Zakir Naik 22/24

There are many sects in Islam. Which one should be followed?
Which madhab to follow?

Is God cruel because he let Adam's sons, Cain and Abel kill each other?

And hold fast, all together by the rope which Allah (stretches out for you), and be not divided among yourselves; and remember with gratitude Allah's favour on you; for ye were enemies and He joined your hearts in love so that by His Grace, ye became brethren; and ye were on the brink of the pit of fire, and He saved you from it. Thus doth Allah make his signs clear to you: that ye may be guided. [Al Quran - Chapter 3, Verse 103]

Who is better in speech than one who calls (men) to Allah, works righteousness, and says "I am of those who bow in Islam"? [Al Quran - Chapter 41, Verse 33]