How do I add autocompletion for variable names for a new language? [duplicate]

Question

I'm writing code in a language called choicescript, and it defines global variables like so:

*create <variable name> <initial value>

Global variables are defined only in a single file named startup.txt, and their scope is all files in the project.

Local variables are defined like so:

*temp <variable name> <initial value>

Their scope is only the file in which they're defined.

Examples of variables being used in the code:

*temp str "Dogs"
*set str "Cats"
I like ${str} the most.
*if (str = "Dogs")
    I like dogs the most.

Choicescript also has labels for the *goto command. Example:

*if str = "Dogs"
    *goto dogs

*label dogs
Some text about dogs here.

Scope of labels is the file in which they're defined.

What I'd like to be able to do is for Vim to maintain a list of variables and labels for autocompletion. How can I set this up?

Here's the syntax file I created (which is still doesn't contain folding and is still a work in progress since my Vim experience is still noob level):

if exists("b:current_syntax")
    finish
endif

let b:current_syntax = "choicescript"

setlocal iskeyword+=#
setlocal iskeyword+=*

set breakindent

syntax match csOption '#.*$'

syntax keyword csCond *if *else *elseif *elsif *selectable_if 

syntax keyword csRare *delete *allow_reuse *goto_random_scene *share_this_game *link *more_games *print *script *check_achievements

syntax keyword csGoto *ending *finish *gosub *gosub_scene *goto_scene *goto *gotoref *return

syntax keyword csChoice *choice *fake_choice

syntax keyword csReuse *disable_reuse *hide_reuse 

syntax keyword csSet *create *temp *set *setref 

syntax keyword csOther *input_text *input_number *image *line_break *page_break *rand *stat_chart *achieve *achievement

syntax keyword csLabel *label 

syntax match csComment '^\s*\*comment.*$'

hi def link csOption    Special
hi def link csCond  Statement
hi def link csRare  Todo    
hi def link csGoto  Constant
hi def link csChoice    Identifier
hi def link csReuse Type     
hi def link csSet   Keyword 
hi def link csOther PreProc
hi def link csLabel     Function
hi def link csComment   Comment

hi Comment guifg=green gui=italic
hi Identifier gui=bold 
hi Constant gui=bold
hi Statement guifg=blue
hi Function guifg=orange gui=bold
hi Keyword guifg=maroon gui=bold
hi Special gui=bold