Hi Kas - this depends on your budget to some extent. I started out using a microsoft built in Photo Editor (this struggles with large files) - which I think was bundled with Windows 2000 onwards.
I then used a product called Photo Impact which came free with a photo magazine. I believe the older version of this software are still downloadable free - just the latest version (10?) which still costs. This has a built in photo album to organise your pictures which I also found useful then.
Take a look at this picture it was edited using Photo Impact.
http://www.worldphotographyforum.com...500&ppuser=818
There are numerous others - if your PC is recent it may have even been bundled with Microsoft other photo editing suite. I can't recall it's name.
Other packages which rate very highly are Photoshop - one you'll probably here the most about and is available in several versions, agan depending on pocket and Paint Shop Pro formerly by JASC now owned by Corel.
All these packages contain a feature which enables you to edit the background, usually by picking a colour, i.e. in your photo the grey of the road, and having the software look for all pixels containing that colour and removing it. It's a bit more involved than that but essentially that's how it works.
Most of these packages explain some of the functionality within their respective 'Help' menus.
I'm sure once you've sorted a package that suits you and you repost any specific questions here about it you'll get a host of responses.
Jon