Converting Simple strings

The easiest way is to use the strconv.Atoi() function.

Note that there are many other ways. For example fmt.Sscan() and strconv.ParseInt() which give greater flexibility as you can specify the base and bitsize for example. Also as noted in the documentation of strconv.Atoi():

Atoi is equivalent to ParseInt(s, 10, 0), converted to type int.

Here's an example using the mentioned functions (try it on the Go Playground):

flag.Parse()
s := flag.Arg(0)

if i, err := strconv.Atoi(s); err == nil {
    fmt.Printf("i=%d, type: %T\n", i, i)
}

if i, err := strconv.ParseInt(s, 10, 64); err == nil {
    fmt.Printf("i=%d, type: %T\n", i, i)
}

var i int
if _, err := fmt.Sscan(s, &i); err == nil {
    fmt.Printf("i=%d, type: %T\n", i, i)
}

Output (if called with argument "123"):

i=123, type: int
i=123, type: int64
i=123, type: int

Parsing Custom strings

There is also a handy fmt.Sscanf() which gives even greater flexibility as with the format string you can specify the number format (like width, base etc.) along with additional extra characters in the input string.

This is great for parsing custom strings holding a number. For example if your input is provided in a form of "id:00123" where you have a prefix "id:" and the number is fixed 5 digits, padded with zeros if shorter, this is very easily parsable like this:

s := "id:00123"

var i int
if _, err := fmt.Sscanf(s, "id:%5d", &i); err == nil {
    fmt.Println(i) // Outputs 123
}