Well if you want to install a new sound card you simply need to install it into the computer and plug in your speakers to the new sound card. It is unnecessary for you to disable the sound card integrated into your motherboard.
However I would make one suggestion before you go out and buy a new sound card. That would be to make sure that your integrated sound card is truly the problem. Test your speakers on another computer, and also try updating the drivers of your sound card and see if that fixes the problem. Drivers can usually be found on the website of whoever you purchased your computer from (i.e. Dell, HP, etc).