Q:

Addition of two numbers by taking user input

How to add two numbers by taking user input?
(println "What's first?")
(let [num1 ( read-line)]
(println (str num1)))
(println (str num1))
(println "What's sec?")
(let [num2 (read-line)]
(println (str num2))) 
(println str(+ num1 num2)))

A:

(let [num1 (do (println "What's first?") (Integer/parseInt (read-line)))
      num2 (do (println "What's sec?") (Integer/parseInt (read-line)))]
  (println (+ num1 num2)))

do blocks return result of evaluation of last expression. Then parse the result of read-line to a string and assign it to respectively num1 and num2, then add the numbers and print the result. 
Note there's no need to convert the result to a string before passing to println as you tried. If you would like to do it the str should be within the parentheses: (str (+ num1 num2)).

