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