If you want a good explanation the better way is google it by entering keywords like "joomla template creation" or "joomla template creation tutorials".
I could explain you , but for Joomla 1.5 template. If you couldn't find anything better for your choice, then I may post a thread under tutorial section on forthcoming days will be titled as "how to create Joomla 1.5 templates".
YOU MAY NEED THESE......
*** You may need a WYSIWYG (What You See Is What You Get) Editor for easy creation.I used MS frontpage express before . Nowadays there are no frontpage available in the market, but similar software " Microsoft expression web" available. (you may need to buy it)
*** A local php server : wampp server (apache,MySQL, PHP) , where you need to install Joomla 1.5. Wampp is free to use.
.................I will Explain remaining in tutorial section later................