Welcome to my first tutorial post on this site

Okay let's start with some easy things;
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function hit(part)
print(part.className)
end
script.Parent.Touched:connect(hit)

Explaintion; The first line is naming a function to ´hit´ and naming the arguments ´part´ (in this touched Script the arguments will be the part that touched it).
The second line will print the part´s className (The part whoever touched it. to see what it print try this; View>Output)
The tirth line will end the function. Note; end´s are needed for an if statement, a function and do (such as for......do and while .... do)
The fourth line is epicley a must in this touched Script; when the script´s parent is hitted|touched then it will run the function name in the connect arguments so we named a function earlier to ´hit´ and when script.Parent is hitted|touched then it will run that function.
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Anoymous functions.
The Anoymous functions are pretty much the same ash normal functions, but there are changes;

script.Parent.Touched:connect(function(p)
print(p.className)
end)

This will work pretty much ash the normal functions, expect they don´t have a name, the connection is in the begin and the function end needs an ´)´. (only the function!)

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Calling functions

Calling functions is simple, for example;

function classname(p)
print(p.className)
end

classname(Workspace.myrco919.Head)

Now we name a function ´classname´ and the argument ´a´.
The second line it will print a´s className
The tirth line will end the function.
The fouth line; The suprising part is here, we called a function the ´classname´ (function name) my head wich will result into `Part´ since my head´s className is part.

Well this was my first guide, hope it helped.

Sorry I made a small mistake;

function classname(p)
print(p.className)
end

classname(Workspace.myrco919.Head)

Fixed it for you ;)

Tough it's very usefull calling a function, mostly it decreases the amount of time that is needed;

local door = script.Parent

function open()
door.Transparency = 0.5
door.CanCollide = false
wait(2)
for i=0.5, 0, 0.1 do
door.Transparency = i
wait(0.3)
end
end

door.Touched:connect(function(p)
local h = p.Parent:findFirstChild("Humanoid")
if h~=nil then
if p.Parent.Name == "myrco919" then
open()
elseif p.Parent.Name == "naknak" then
open()
end
end
end)

I like to only make multiple functions when I need to do the same thing more than once.

Altough I have another example;

local names = {"J0HN"}

function check(ietm)
for _,v in pairs(names) do
if ietm.Name == v then
return true
else
return false
end
end
end

game.DescendantAdded:connect(function(p)
if check(p) then
p:remove()
print(p.Name.." Was a fire virus.")
else
print(p.Name.." Wasn't a fire virus")
end
end)

You could just make that into one function.

local names = {"J0HN"}
game.DescendantAdded:connect(function(p)
for _,vir in ipairs(names) do
if (p.Name == vir) then
p:remove()
print(p.Name.." Was a fire virus.")
else
print(p.Name.." Wasn't a fire virus")
end
end
end)