To install your chosen operating system, and still have plenty of free space for files and data, buy an 8GB or SD larger card. Operating system downloads and installation instructions are available on the Raspberry Pi Foundation website.
Unfortunately, not every SD card on the market is compatible with the Raspberry Pi. So, I'd suggest consulting a compatibility list before making a purchase. One of the most comprehensive is provided by the eLinux website.
Visit my Raspberry Pi page for news, reviews, advice and tutorials.