help...seeing web server from anywhere on the internet

there is one simple matter I can't figure out. If I connect an
ethernet enabled micro to my home network and assign it
an IP address I can access its web server from my PC.
But, my firewall has it's own IP address assigned from my
ISP...How do I access the micro from outside, somewhere
else on the internet?
(I tried searching for the answer but came up with a million
unhelpful links)