In basic idea 5th edition is very, very simple; instead BAB, skill ranks or base save bonus you got profiency bonus (which is based on your class level). You use profiency bonus to lot of things and thus it's most important stat to keep in mind firstmost.
Then you add any stat modifiers and with every class you get 2 saves to have profiency bonus in (and can take more via feats). Saves are all the six stats what you do roll with every edition anyways (and you add stat modifier for all saves, with or without profiency bonus). AC is based on armor you wear + dex modifier. Every armor gives base armor modifier (and unarmored bonus is 8 what I remember). As caster you can wear any armor as spellfailure with armor is not a thing anymore (it's more on question do you got hands free for casting and this can be overcome with war caster feat).
Then there is concept of short and long rest; short rest allows recover certain class features and recover HP based on how many HD you spend in short rest (and warlock's case all spell slots spend and wizard's case single spell slot) while long rest recovers everything.
Zaer covered the basics, but I can break it down even more: mechanically, a character is a combination of Race, Class, Background, and Subclass, all choices you can make in any order. Grab your class and background starting gear, pack personality traits/ideals/bonds/flaws, note your racial and class abilities, and go. You don't even have to take Feats anymore, as a matter of fact, so there's another layer of complexity you can safely ignore.
As for the derived traits, it's all pretty damn simple, and there's little need to optimize because of 5E's tight action economy. As far as optimization goes though, in 5E it's better to be a generalist than to be overspecialized, since bounded accuracy means you get more benefit from every bonus you add to a roll, but it also means that when your bonus is too low, you feel it. To that end, focus on two key ability scores for your class - your main attack stat, and one other supporting stat. For example, every Fighter wants either Str or Dex as primary and Con as secondary. Dex fighters can actually neglect Con a bit and put more points into other abilities like Wisdom to shore up an oft-targeted save.
If you are a primary spellcaster like a Bard, Cleric, Druid, or Wizard, you want your casting stat to be highest (Cha for Bard, Wis for Cleric and Druid, Int for a Wizard) and then focus on your secondary stats - Con is always a good one to get high, but some concepts and builds call for focus on different secondary stats. For example, a Moon Druid can get away with low physical scores because most of his HP and physical capability will come from Wild Shaping into beast forms.
Specialists like Rogue and Warlock are trickier - Rogues universally need a high dex, so don't start with less than a 15 there if at all possible, and 16 or higher is better if you can manage it. Dex is everything for a rogue - it's offense, defense, and skill bonus for your most crucial abilities all in one. Warlocks are an odd duck because they can perform any party role except healer with the right skill, spell and invocation coices - a warlock who wants to be a party tank should go pact of the blade with the fiend patron and take the spells Hellish rebuke and Armor of Agathys with the Fiendish Resilience and Thirsting Blade invocations, and actually have decent Str or Dex (Depending on weapon choice) and con like a fighter would with Charisma as a middling to stat since he won't be using any spells dependent on Save DC, though he might want to eventually get it decent for Lifedrinker later on.
I could go on forever, but you should get the gist - complexity in 5E is up to you. if anyone wants help turning a concept into a mechanically viable character (my specialty) or just understanding something about 5E, let me know. I'm happy to help.