1) Don't do it in flash!! Small parts in flash, sure, but otherwise it's a terrible idea.
2) You need to buy hosting for your website (some computer has to store the site somewhere), and you also need to buy a domain name (the dotcom address).
That's basically all you need. It gets more expensive if you want to let people buy from your site. This involves setting up a system to process the payments, as well as paying for a certificate for your site, etc. It's why smaller companies tend to just use Paypal or similar.
you can buy your domain from somewhere like this...
http://www.123-reg.co.uk
and hosting is available from many places. My host is Joyent, but Dreamhosts are ok, and lots of cheaper places exist (web-mania.com for one).
once you have those two, you go into your domain control panel and change the DNS entries to point to your host's DNS servers, so that your website appears when you type in its address.
to send your files to their computer to put online, you use a ftp client such as filezilla.