So I understand the the basics of a Concentration check the only thing that I don't understand is what, if anything, do you add to the D20 roll? I can't find anything in the book so any assistance is helpfull.
Technically there's no such thing as a "concentration check" in 5e.
A "concentration check" is actually a constitution saving throw.
From page 203-204 of the PHB:
Taking damage. Whenever you take damage while you are concentrating on a spell, you must make a Constitution saving throw to maintain your concentration [...]
The DM might also decide that certain environmental phenomena, such as a wave crashing over you while you're on a storm-tossed ship, require you to succeed on a DC 10 Constitution saving throw to maintain concentration on a spell.
As such, you would add your constitution modifier and proficiency bonus, if proficient in con saves, to the D20 roll.