Maintaining privavcy / secure deletion of files/data - how can this be achieved?

Greetings

I was recently party to a converstaion which I found disturbing, in that it centred around data security and privacy. I was amazed at how many people think that highlighting a file and clicking delete means that it is gone. I am also amazed at the multitude of opinions out there as to what what needs to do in order to secure one's self in terms of deleting unwanted files and removing all traces of internet activity. Rather than blindly follow on what I ah heard, I thought I'd post a question here and get some proper guidelines/advice.

I suppose my question(s) would be:

(1) How do I ensure that a deleted file is deleted (cannot be unerased by the million tools out there) ?
(2) How can I ensure that if I format a harddrive, the data can't be recovered?
(3) How do I remove all traces of internet activity?
(4) How do I protect myself against internet attacks?

I think that's about it (if there's anything I';ve left out .....)