Hi everyone. I am new to the world of d & d, I have the player manual and the DM manual, but I cannot find this information. Maybe it's so obvious that I didn't notice. I would like to know how to calculate a player's degree of challenge. Thank you. 

Here's the info that you asked for.

"A monster's challenge rating tells you how great a threat the monster is. An appropriately equipped and well-rested party of four adventurers should be able to defeat a monster that has a challenge rating equal to its level without suffering any deaths. For example, a party of four 3rd-level characters should find a monster with a challenge rating of 3 to be a worthy challenge, but not a deadly one."

If you are referring to challenge ratings, the Monster Manual has this to say:

A monster’s challenge rating tells you how great a threat the monster is, according to the encounter-building guidelines in chapter 3 of the Dungeon Master’s Guide. Those guidelines specify the numbers of adventurers of a certain level that should be able to defeat a monster of a particular challenge rating without suffering any deaths. An appropriately equipped and well-rested party of four adventurers should be able to defeat a monster that has a challenge rating equal to its level without suffering any deaths.

You can safely assume that a player can stand toe to toe with a monster with CR = one-quarter of the player's level, and be challenged by a monster with CR = half the player's level. More than that and you are likely in for a short fight.

Yes, players themselves don’t have a Challenge Rating, but it’s a rating of NPCs that is compared against their player level.