How To Proceed

I built a web site garybuyshomes.com using the raindrops theme which is a 3 column theme. I have been reading a little about CSS, Php, childthemes etc. I know very little but want to learn. I want to do things such as have pages with no columns or just one column (need 3 for the home page) and many more little things. My plan is to build a new site with a different URL then transfer the new site when done. My plan is to learn to back up a site, then learn about child themes to use for the new site so I can update with no problems then start to modify the copy of the existing site. I am new so if any one would care to criticize my plan or offer any suggestions I would appreciate a push in the right direction.