If you wish to explore skinning then also have a look at as this is where we begin explaining how skinning works in DotNetNuke.
I think it would also be worth your time installing a test version of DotNetNuke on your local computer, this way you can experiment with using DotNetNuke, learn all the details of how DotNetNuke works without risking messing up your live website. I think you will find it much easier and quicker to learn in this way and then you'll know what to look for on your live website.
If you decide to install DotNetNuke to your local computer, I would recommend you do this within a virtual PC:
How to setup and use Microsoft Virtual PC 2007And then have a look at these tutorials, depending on which version of Windows you are using:
You will probably want to install the same version of DotNetNuke that your website is currently using, you can find out which version of DotNetNuke it is using by going to the host menu / HostSettings
Hope this helps you to get you started, if you have any further questions, please do not hesitate to ask,