How to make a prime number generator using Ruby?

Easy.

# Defining the function with two parameters
# number of primes between m and n.
# (Note: It still needs to calculate all primes
# up to m even though it won't be printed on the
# terminal.)
def prime(m, n)

  # This holds the total number of primes in
  # the range specified in the parameters.
  number = 0

  # This is the array to hold all the primes.
  ary = []

  # This loop from 1 to n runs until it reaches
  # the n specified in the parameter.
  for i in 1..n

    # Unless it's divisible by 2
    # (*psst* even numbers);
    # this will exclude 2 as a prime
    # but I'll put it back it after
    # this  block.
    unless i % 2  0

      # This count shows if the current
      # number is divisible by a number
      # other than 1 or itself.
      # (This actually should be a boolean.)
      count = 0

      # In the prime array each element is
      # taken out and put in j, then i is
      # mod j to see if it's divisible by
      # some prime number, there for making
      # a composite number.
      ary.each do |j|
        if i % j  0

          # That is unless the prime number
          # is 1 or itself.
          unless j  i || j  1
            count = 1
            break
          end # End of 'unless j  i || j  1'.
        end
      end

      # If no prime number is divisible by it
      # push the prime into the prime array.
      if count  0
        ary.push(i)

        # Unless the prime is not in the range
        # the parameters specify, don't print it.
        unless i <= m
          puts i

          # Add to total number of primes.
          number += 1
        end
      end
    end # End of 'unless i % 2  0'.

    # This is to add in 2 from the
    # exclusion before.
    if i == 2
      ary.push(i)
      unless i <= m
        puts i
        number += 1
      end
    end
  end # End of 'for i in 1..n'.

  # Display total number of primes.
  puts "number #{number}"
end

# Run the function using input from the terminal.
prime(ARGV[0].to_i, ARGV[1].to_i)

Save as prime.rb run it and enjoy.