I am new here, I would like to know

thanks

There's really no definitive answer to this question. The devs have not made known the actual skills needed for each position. It's been up to the players to try to figure it out and even after 11 years, there's still some disagreement to an extent.
Some are (or should be) pretty obvious. Your QB should need passing, WR should need catching, defenders should need tackling, etc. Outside of that, it's trial and error and/or guessing. Jaythegrim's post link above has some good educated guesses and could even be completely correct, but again, nobody knows for absolute certain.
The only thing the devs have said on the matter is that each position requires something like 3-5 primary skills and a few secondary skills. But even then, there are some who think this may not be completely accurate in every case.

Good luck and welcome to GI.

Not the same as my own preferences list, but it looks awesome, not to mention it's straightforward to use. Thanks for posting the link