Basic JASS spell tutorial?

I would like to find some tutorial for basic JASS spells and how is it better than using GUI although GUI can do the same thing.
I would also like to know what is the "custom script code" text area for. How do i actually use it? Normally i see alot of JASS spells uses that area with long scripts. What is the main purpose for it? How does it link to the fuction?

I want to create a spell that it damages unit in a cone (carrion swarm), but deals #damage * hero attribute. Its hard to do in GUI, can anyone teach me how to do it using JASS?

Check Daelin's and Vexorian's JASS tutorials. Your spell damaging in cone wouldn't be much easier in JASS I think. Easiest way is to use Carrion Swarm/ Breath Of Fire and just damage units with buff (Carrion swarm leaves a buff).

Custom script code is field for global JASS functions or variables, that are used by many functions inside the map. Mostly you'll find in this field Handler Vars because they are necessary in many spells to pass local variable into timer or trigger function.

GUI can not always do the same thing, or, it can, but in an incredibly difficult and inefficient way. The custom script section is where you put all your shared Jass functions - functions that multiple of your spells use. You can have whole systems there that support your spellmaking and make it easier, from the most basic Handle Vars to the extensive Vexorian's Caster System.

For coned spells, you can take a look at the cone spell template made by Vexorian, you can find it in his Spell Template Pack or you can make one yourself, once you get more experience with Jass; if you haven't done so yet, go through the Jass tutorials on this site, afterwards, you can take a look at how Handle Vars work and then you should have the basic knowledge for Jass spell making.

Isn't Vexorian's Spell Template Pack using Caster System?

Most likely, I don't know. Does it matter?

Wait... I remember something like he made templates without cs also... or maybe it wasn't Vex... Well, if he wants to import Caster System it doesn't matter.

Latest templates system use caster system for events and still needs the template data registering functions and that stuff too.

Most of the templates needed the caster system anyways

you can always learn from jass spells other people made.