How to convert string into numbers?

I think there was something called tonumber()

So, yeah. How do I use tonumber()?

Learn to use documentation:

A simple search on the Wiki will result in this page being found:

http://wiki.roblox.com/index.php?title=Function_dump/Basic_functions#tonumber

tonumber is a function that takes in either one or two arguments.

When given one argument, it returns the decimal number corresponding to the string given, or it may return nil if the thing given isn't a number:

print( tonumber( "1" ) ) -- 1
print( tonumber( "1." ) ) -- 1
print( tonumber( "1.5" ) ) -- 1.5
print( tonumber( "-5.1e3" ) ) -- -5100

print( tonumber( "5kg" ) ) -- nil
print( tonumber( "" ) ) -- nil
print( tonumber( "cat" ) ) -- nil

The second, optional argument, is a base. E.g., we can convert from binary:

print( tonumber( "101", 2 ) ) -- 5
-- since 101 in binary is 5