Example Ruby miner. The way Bitcoin miners try to find a hash for a given complexity and transaction

miner.rb

# Mining example
require 'benchmark'
require 'openssl'

def hash_ok?(hash, complexity)
   hash.start_with?("0" * complexity)
end

# Appending a nonce the the 
def create_target(str, nonce)
    "#{str}-#{nonce}"
end

# Hashing with standard SHA256
def digest(str)
    OpenSSL::Digest::SHA256.new(str).to_s
end


# Default number of leading zeroes in the target hash
# The more zeroes, the bigger the complexitiy 
COMPLEXITY = 3
complexity = COMPLEXITY
msg = "Hello, world"

# Custom message from the command line
if ARGV.count > 0
    msg = ARGV[0]
end

if ARGV.count > 1
    complexity = ARGV[1].to_i
end

puts "-" * 100
puts "Target complexity is #{complexity} leading zeroes in the hash"
puts "-" * 100
start = Time.now
i = 0
loop do
    t = create_target(msg, i)
    d = digest(t)
    print "trying: '#{t}', hash: #{d}, nonce: #{i} "
    break if hash_ok?(d, complexity)
    puts "[BAD]"
    i += 1
end
finish = Time.now
puts "[GOOD]"
puts "-" * 100
puts "Target is satisfied after #{i} hashes"
puts "Needed #{finish-start} seconds to do it"

How can I mine BTC with it?

It's just a demonstration about the core concept, unfortunately you can't mine anything with this alone.