If the other answers make sense I apologise for how patronising my answer must sound – but if they don’t make sense perhaps my N00B style answer will 🙂
You can’t mess up the installation of wordpress on your live server if you are simply developing a theme on the localhost installation (a skin is known as a theme when it comes to wordpress)
Once you have developed the theme on localhost you then Zip the theme up and upload it through the themes management menu in your live wordpress installation. There are other ways to do it, but I find this way far easier and less prone to user errors.
BUT – you don’t do it by downloading everything from your live site to localhost
try reading this page http://codex.wordpress.org/Installing_WordPress_Locally_on_Your_Mac_With_MAMP
or google “install wordpress locally MAMP“
This should walk you through the steps to install it
Creating the theme you want – that’s a whole other kettle of fish, but again try using google to look for some tutorials to help you walk through it – or find a theme that you love that someone else has already created
Hope this helps even if only a little