I am using Microsoft product for years. Now I would like to switch to free Linux. I don’t know anything about this operating system. Can anyone suggest me from where can I download free Linux and which is the best one.

Try Ubuntu, if you don't have a spare HD you can test it from a Live CD. Having it on another HD is the best way to go if you are just starting to use Linux.
Other distributions are just as good, it's just a matter of picking the right one for your needs. This site gives you a short tour of the major ones.
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