Please start new threads for new questions.
What browser? Every browser does this differently.
Go to Safari > Clear Browsing Data...
Safari main menu at the top.... History, clear history. If you press reset Safari it will do more than just wipe the browsing history so be careful doing that.
Whoops, I was thinking of Chrome (I was away from my Mac). You want the reset option.
How often should you clear history? As often as you'd like. However, there isn't really a reason to do it for most people. Any particular reason why you're doing it?