This is by far the most important thing in PoE. and how far you can go.
After complete Act 10, the game just get started. the Maps from T1-16, without a competitive build. very few can go pass t10+

How Top Player Design their build?
There are several way to start

1) Pick Main Skill, Pick eng game item, Pick Passive, Pick Jewel, Design your defence
most people start with skill. and see how far the skill can boost till the end game with correct item, passive, and defence.

example one of the popular Cleave Build

Pick Cleave
+ Overwhelming Odds (Jewel ) ( everytime you cleave = you fortified )
+ Juggernaut Ascendancy Passive, Fortify get stronger, fortify boost dmg as well
+ Perseverance Belt, When you fortify, you gain onslaught

You need to find the line that keep trigger one another. and snowballing the damage.

Just No. Time wasting. i try new build from scratch once a week. SSF just damm too slow

1. DO NOT GRIND. Yes. the more you grind, the slower you are. JUST complete the objective as FAST as you can, dont stuck at low level map
2. If you found boss are too hard. you just have wrongly design your char.
3. What is the purpose of respec point? Having 2 build in a Char ( early rush type, end game type )
early game = more offensive passive
end game = balance
4. DO NOT SPEND CURRENCY on crafting in early game. you just dont know what you are doing.
except low level essence.
5. Use currency to trade.

There are too many tools to ease your time and efficiency :
path of building ( design your build )
pathofexile.com/trade (trade database )
poebuilds.cc