Ruby if-statement with strings always go to first case (not rails)

I'm quite new to Ruby. So I have a method that takes string input and then compares it several times. The problem is - no matter what - it always go to the first case. For example:

def method (s)
    if s <=> "help"
        help()
    elsif s <=> "add"
        puts"Enter 2 numbers"
        a = STDIN.gets
        b = STDIN.gets
        add(a,b)
    else
        err("Invalid command, type 'help' for list of available commands")
    end
end

class.method(STDIN.gets)

always do "help()", no matter what the input is. How it should be written?