Intro To Homebrewing
Forging A Path
First things first: no guide can possibly capture every possible nuance of PoE character building. To do so would require not only a comprehensive list of every mechanic in the game - already far beyond the scope of this guide - but also lists of all their possible interactions. This section contains guidelines, not rules, many of which might not even apply to an experienced player using high-end equipment. Individual pieces of advice here can easily be wrong for certain builds, and sometimes very obscure mechanics can make a build.
If you think I'm kidding about the level of potential complexity involved, consider this: one player managed to create a build that killed enemies by swapping weapons. Not by attacking, not even by being attacked - by sitting there and spamming the weapon-swap button. How did they do this? Well, they used a weapon that auto-summoned a few minions (minions that die on weapon swap) with Heartbound Loop, a ring that causes damage to you when your minions die, to deal damage to themselves by swapping. They then used various other mechanics (in several variants of the build) to trigger skills by taking damage.
You are perfectly free to leave the beaten path in this section. Just keep in mind that the more moving parts you have in a build, the more likely it is that as a new player you've misunderstood one of them.
Get Path of Building
Path of Building is a pretty much mandatory tool for any serious Path of Exile theorycrafting. It's a comprehensive calculator for almost all of PoE's mechanics, in addition to an interactive passive tree, skill gem, and item database. I use it when creating my own builds, as does every other player remotely interested in optimizing them. In fact, it's so comprehensive that its developer was actually hired by Grinding Gear Games, the company that created Path of Exile in the first place.
You know how I said a few paragraphs ago that there are no hard rules? I lied. "Get Path of Building" is a hard rule.
PoB's developer is busier today than he once was, so some information in PoB is out of date. Be careful, and double-check anything critical to your build (especially item stats).
Think About Flavor
There are lots of possible "feels" to a build in Path of Exile. Consider what you're looking for to help narrow down your options. Do you want to play a ranged attacker who runs around quickly? Consider a Deadeye build. A minion-summoner with a giant mobile army of skeletons? You'll want minion bonuses. The number of options can easily be overwhelming, and thinking about a general sense of what kind of build you want to play is a good way to narrow them down.
Creating your own build requires a very strong grasp of Path of Exile's mechanics in most cases. In particular, make sure you understand the critical difference between "More" and "Increased" as discussed in an earlier section; most builds are built to some extent around abusing More modifiers in one form or another.
You may also want to consider reading through the Mechanics Reference section in its entirety before moving on.