Google is your friend: Be nice and visit him often. He can answer questions a lot more quickly than posting them here...
A very quick search
using your subject as the search term gave over 100,000 hits:
F12 Developer tools in browser - Google Search[
^]
So start there, read some of the links, and give it a try!
In future, please try to do at least basic research yourself, and not waste your time or ours.
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