This is certainly a broad question. A site that would be useful to everyone would cover everything from A to Z in the dictionary. The first thing to narrow down would be the scope of who you want to help.
Are you wanting to help computer technicians, webmasters, car owners, etc.?
I suggest that you start a web site about something that you're interested in. It would also be helpful if you're knowledgeable about the subject of the website.
hmm, thanks. well im interested in pc technology a lot! Sports also. big soccer player. I also designing, learning 3d right now. I like photoshop a lot too.
Make a website about something that you are interested in. Focus on one particular topic and put all of your knowledge about the topic onto your website.