First off, you need to know what the default ftp folder name is. Often it's public_html or www, etc.
That's where you ftp your files (after extracting them onto your local drive).
If you have done that and put them into a folder named /public_html/joomla/ (or whatever), you should be able to browse to
http://yourwebsitename.com/joomla/ and start the setup and configuration process.
If you've done the above properly and are getting a 404, then, well...I would use my ftp program and have a look to ensure that there is an index.php file in the /joomla/ folder.
if it's there, you shouldn't be getting the 404, unless your website is not setup to handle php or your .htaccess file has done some extension revamping.
Most hosting servers that are really Joomla friendly will offer automatic Joomla installs that are totally hassle free.