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Office.com
Chandoo.org
easy-excel.com
Youtube channels
youtube .......................................
I suggest YouTube but filter it out for number of views to get the most viewed ones.
I do support Mr. Kapadia's answer
You can browse on you tube or on its website tutorials
Go for Lynda.com
They will teach you step by step in very systematic way.
So many kind of site available in google search such GCF learn free organization and ALISON ,Utube etc....