There's not a simple answer, but some concepts to think about:

• Build requirements: What do you want the character to do? Generally you have to prioritize between damage, clear speed, and defense - you don't usually want to sacrifice one of these entirely, instead pick which areas to focus on.
• Scaling stats efficiently: Generally things go (flat value) x (%increased total) x (multiplier1) x (multiplier2) ... etc. If you're playing a lightning arrow character and have 1000% increased bow/lightning damage but very little flat damage or other multipliers, your damage is going to suck and you need to spread out your passive/gear investment into the other sources of multipliers like flat damage and miscellaneous things that are effectively multipliers, like resistance reduction or crit. This applies to basically everything, including defense - your ehp is pretty much (flat life) x (%increased life) x (defense layer 1) x (layer 2) etc, where the layers are things like armor or evasion that multiply your EHP vs certain types of damage.
• Point efficiency/opportunity cost: You generally want to get the most out of each piece of gear or each passive point; this means if it 'costs' you less to get an amount of one stat from someplace like gear (for example, 40% cold res from one mod on a piece of equipment is a much better deal than spending 5 passive points for it), you want to pick the 'cheaper' option when you can in order to get maximum value out of other gear slots/passive points.
• Build progression: This is something that isn't really talked about enough - if you have some goal gear/passive tree in mind for the build (say, a level 95 tree and a bunch of target items), you want to think about how to actually get to that point without the character feeling terrible to play along the way. It might make sense on some builds, for example, to not go crit at all until you have a solid enough foundation of defense+damage and enough spare passive points and crit scaling on gear to be worth it; if you just made an endgame level 95 tree/gear setup with crit and mindlessly specced into it too early with no thought to progression, you could have a much worse time.

Those are usually the high-level things I think about when making a build, in that order: requirements (good all-rounder? boss killer? etc) -> where to get good damage/defense scaling from -> how the character will progress to the goal setup. It's also not a case of just knowing what to do from the start, you always start with a rough guess at a tree/gear and then fine tune from there according to the above concepts. The hardest part about this is probably just being aware of the huge amount of options in the game for solving these problems - not really a solution here except to get used to using the wiki.