It's actually not that hard! There's only a handful of core rules to know for every session and the rest you can learn as you go.
Attacks and spell attacks: 1d20+prof (proficiency bonus)+ability+special bonuses (items, buffs, etc)
Ability checks: 1d20+ability. If it's a skill check you have proficiency in, add +prof.
Saving throws: 1d20+ability, +prof if you're proficient in that saving throw.
Note: all of these bonuses are summed up on your character sheet under your spellcasting page, your weapons, skill lists and saving throw lists.
In encounters you can do the following: Action (extra attack included in 1 action), Bonus Action, Reaction, Movement, Item Interaction, and any number of Free Actions.
DCs for figuring out how hard something easy:
5: very easy, most people can do this most of the time
10: easy, people trained can reliably do this
15: medium, decent odds if skilled
20: hard, rare success unless very skilled
25: very hard, rare success even with highly skilled
30: nearly impossible, heroic aptitude still fails most of the time
35: godly, the highest DC likely to see. impossible without epic amounts of skill and even then very unlikely. even demigods may fail
This argument is bad faith. Or you're just plain wrong. You can pick.