How do I use a var as a string?

[Tom Browder <tom.browder@gmail.com>]

I want to bind a string to a variable and then use the variable name as the
string when a string is expected.  I have tried variations of this:

(defvar my-str (concat "my " "string"))
(print my-str)

and get an error about void symbol my-str.

I have tried putting an apostrophe in front of various places but haven't
yet found a working solution.  So how can I define a variable whose value
is a string and then access the actual string value by using the variable
name?

Thanks.

Best regards,

-Tom