Noob com user here. i would like to know how to reformat my laptop. Does anyone has guide for reformat? Do i need to backup my files first or during the reformatting period? My laptop is asus X42J series
Your laptop will come with a manual, it should be found in the ASUS help file on your laptop. To answer the question as to whether you should backup the files, it depends on whether your files are on the primary partition that holds the OS or on a separate partition. Note also that you should backup anyway, regardless of whether you want to reformat your laptop or not. It's good practice.
Next, if you are using Win 8, you might want to consider a refresh of the OS rather than a reformat. I guess you want to get the laptop to run faster cos it's starting to slow down. Doing a refresh to reset all the OS's registries, drivers, etc would certainly help. And a refresh will not erase your files.
You can read the Windows Help file for the various options available now on Win 8.