API is less important when getting started. Syntax is the most fundamental. All keywords in lowercase (function, loop, if), and before learning blizzard.j and common.j API it helps to know all those functions and natives are CapitalizedLikeThis. I have happened to learn a lot of the API just by doing, but I only have 20-30% memorized. I look up functions and natives when necessary, and sometimes I am pleasantly suprised with what I find.
Before I learned JASS, I had done a lot of work in HTML and BBCode. They are not strong languages, but I knew basic concepts going in: when you open a block, you need to close it. That's one of the most important things going in. There are three main blocks to consider in vanilla JASS, but many superficial ones in vJass which you can read up on in the JassHelper Manual.
Here are the JASS blocks and what keywords would be relevant within them:
function FunctionName takes nothing returns nothing //arguments and return type must be declared
local integer i = 0 //locals declared first. They must be set before they can be read.
loop
exitwhen true or false //The loop runs until the exitwhen condition is true
return //or when a return statement is met
set i = 1 / 0 //or when the thread crashes due to an error
endloop
if true or false then
elseif true or false then //elseif is a neat trick to avoid spamming new blocks
else
endif
return i //if this function returned an integer, or any other type, declared the return value after the return statement
endfunction