How do I add strings together.

Title says it all.
Example (how you would do in python):
"hoi" + "hi" = "hoihi"

I have no clue of how to do this in lua.

"hoi".."hi"

You could do

a = "text"

b = read()

print(a..:D/>/> -- That b is made a capital automatic?..

As Pinkishu said, .. is concatenation in Lua. Python treats + as both addition and concatenation, depending on the situation, but for Lua you have to use .. to concatenate strings.