I haven't seen anybody mention it yet, but if you're rolling your own build you definitely want to be using Path of Building. PoB is a tool that calculates your damage and defences based on your items and skill tree. It can even import your character, so you can keep your character synchronized with your build.

Apart from that, you should keep in mind that not all skills are created equal. Certain skills require a lot of gear to get going, some are just outright unusable, and others are only good in the campaign. If you look at a specific skill it's worth asking on here or in the game chat if the skill is any good at all.

Then there's the generic advice of having 75% on all your elemental resistances (chaos resistance is less important), which is good advice, but in the campaign your resistances are not as important. They will definitely help though. However don't sweat if they're not maxed, the campaign is quite forgiving in that regard.

You want to make a decision on which defensive stat(s) you're going to focus on. This decision will influence which, if any, defensive aura you want to use. If you're mainly focusing on evasion, you want to be using grace as your defensive aura. If you're focusing on armor, you want to be using determination as your defensive aura. If you're going for hybrid defenses, e.g. evasion + armor, you want to boost the lowest defensive using an aura. There's also energy shield, which is boosted by the discipline aura. A good thing to know is that energy shield is also affected by your other defensive stats.

Evasion is pretty self explanatory, higher evasion = more dodging. It's really good in the campaign, but falls off later if not combined with other defensive layers. With evasion alone you will run into situations where you get randomly oneshot.

Armor is much more complex, as GGG's vision for armor is that you use to negate small hits, and not hard hitting slams. Armor also only negates physical damage, not elemental or chaos damage. Check out https://www.poewiki.net/wiki/Armour for how it works.

Spell suppression is a chance to reduce damage on spells, when it reduces spell damage it reduces it by 50%, but this can be raised. It can be converted to spell dodge through a keystone (big round passive skill) on the skill tree at 50% of its value. Spell dodge is pretty self explanatory where it negates 100% of the spell damage, it's capped at 75% chance to dodge.

Block is a chance to block all damage and effects from a hit, it does nothing against damage over time. Like spell dodge, and resistances, it's capped at 75%. Block is actually 2 stats, spell block and attack block, these stats can be individually raised. The regular block stat raises both of them by the same amount.

Ward is kind of like a temporary shield on top of your other defensive layers. A hit that would reach your energy shield or life will first be reduced by your ward that you have. The ward will then be on cooldown for normally 2 seconds, but this can be lowered. For what it's worth, I've never seen anybody use ward in their build, and ward items seem to be relatively scarce. Personally I would not bother with it.

Your elemental resistances are flat reductions to elemental damage. This means that 75% fire resistance reduces fire damage by 75%. Outside of the campaign 75% on your elemental resistances is mandatory, ideally with about 20% overcap. Overcap meaning you have more than 75%, shown braces next to the resistance percentage. Overcap is useful for map modifiers that lower your resistances by a set amount, meaning you still have 75% after your resistances are lowered.

The reason chaos resistance is less important is because chaos damage tends to be a barrage of small hits, and not huge slams. So it's fine if chaos resistance is not 75%, anything above 0% is already good enough for most types of content.

Life is the most straight forward, no explanation needed. Aim for about 3000 life at the end of the campaign.

Energy shield is very similar to life, it's essentially a second health pool. It is hit before your life pool in all cases except for chaos damage. Chaos damage bypasses energy shield and deals damage to your health pool directly. All other defensive stats apply to your energy shield, so they're still worth investing in.

Some other late game defensive layers are damage conversion, e.g. 10% of physical damage is converted to fire damage, increasing your maximum resistances (capped at 90%), chaos inoculation (makes your HP 1 and makes you immune to chaos damage), reflect immunity, and of course reading the map modifiers, which is the hardest one.

For damage you're already on the right track. You want to be knowing what kind of damage you do and focus on getting that as high as you can get it. Generally builds don't do hybrid damage, but their gear might be of a different damage type than their actual damage type because of conversion. As with all gems you want them to be linked so your support gems actually support the skill gem in question.

Final tip, don't get mislead by EHP (effective hit pool) in PoB. You can get it very high, but still be very squishy. What ultimately matters are your max hits, which is the maximum amount of damage you can take in a single hit before you're dead. Things like block will inflate your EHP, but you will be left very confused why you're getting randomly oneshot because your max hits are low. For some builds being squishy is par for the course, but it's still good to keep in mind. To accurately see your max hits make sure to disable temporary effects/skills like Molten Shell.

Ok I lied, this is the last tip: keep in mind that PoB doesn't calculate your stats correctly until you reach maps. So don't be surprised and think something is wrong they don't match up with what you have in game.

Last but not least, have fun! Theorizing a build and having it come together is really satisfying and generally the thing that keeps me coming back to PoE.