How to share files with other computers over internet

OK, apart from the obvious security implications of opening a portion of your hard drive to the public Internet...

Your friend has to know the IP address of your computer (or more practically, of your Cable or DSL modem.) If you have a dynamic IP address with your ISP, this is subject to changing.
Then the modem, and.or the router that is plugged into it has to permit http:// or ftp:// access to your machine.
Also, your ISP has to allow it. Some ISPs block any kind of serving from a residential service.

Your best bet is to set up either a website on an external server (you probably have some free space that comes with your ISP account) and upload the files there (you don't have to link the files to a web page, you can just tell your buddies the URL www. myserver . com / myaccount / myfile1.pdf for an example of an Acrobat file you want them to download).

Or you can ask your ISP to set up an FTP area for you on the server, which could be password protected

Or you could get gmail accounts which are large enough for most files to transfer

Or you can investigate one of the many file transfer services or free webspace accounts and use that (others will likely provide links with their favorites).