even if you dont have a static ip, you can still do it. look for dynamic dns services like no-ip.com
you should be VERY carefull though. remember, you are now opening up your computer to anybody on the internet. if you dont have you server locked down tight enough, someone can hack your computer and put anything they want onto it.
unless your extrememly knowledgeable about security issues involved in doing this, just pay the $3 a month basic hosting costs. its much cheaper than what might happen if your computer gets hacked and maybe resulting in your identity stolen because they slipped you a trojan.
imo, if your asking how to do this, you dont know enough to do this safely.
being selective about who you tell your website address to isnt much help again port scanners either. i wouldnt do it.