I have downloaded and created a site now I want to upload it to the web. Can someone please advise how this is done. I haven't done this before.

Also can I have multiple websites with joomla? or once downloaded can i only have one?

depends on part on your web hosting ...

you need to get the files and the DB on to the server. Files are generally easy, just use something like ftp to copy the files up. DB you will need to create a backup, then get the DB on the server to use those files.

If your local DB has different user/password then you need to update your config to use the one on the server.

you can have as many joomla sites as you like, one per folder. If you going to share the DB with multiple joomla versions then you have to change the DB prefix so none have the same prefix.

If you're looking to basically back up your development site and put it up onto your server, I'd recommend using JoomlaPack to do the trick.

Best place to start would be looking at the documentation for Joomlapack, especially Chapter 3 which will walk you through the process.

Though nicholash's suggestion will work, you can end up in trouble if you're not fluent in phpmyadmin and ftp transferring of a site. Using Joomlapack I've found the time to do the move is considerably reduced, as well as being far more consistent.

My aim is to set up a Joomlapack 'out of hte box' build that I can use to rapidly deploy a base joomla build for my clients. This lets me have all the regular components preinstalled, saving time installing them for each new site.

So you could then use a base install to have multiple sites installed on the one hosting account in different folders - basically by replicating the Joomlapack install over and over again.