How do you get individual lines from a webpage using httpservice?

You can do this:

local http = game:GetService("HttpService") 
http.HttpEnabled = true 
local page = http:GetAsync("https://pastebin.com/raw/78VT3MUS") 
print(page)

It sets HttpEnabled to true so that it's allowed to get the page, then gets that page with GetAsync.

(You have to turn on HTTP requests in your game settings for it to properly work)

edit: SHOOT I DIDN'T READ THE QUESTION RIGHT TRY THIS:

local lineNumber = 1
local http = game:GetService("HttpService") 
http.HttpEnabled = true 
local page = http:GetAsync("https://pastebin.com/raw/78VT3MUS") 
local function parse(s) -- stole this, sorry ScriptGuider
    local t = {};
    for chunk in string.gmatch(s, "[^\n]+") do
        t[#t+1] = chunk;
    end;
    return t;
end;
local foundPage = parse(page)
print(foundPage[lineNumber]) -- line1