Work from what you know and have a passion for. Unless you care about the topic, the site won't go anywhere, and will just end up in the wasteland of the internet.
Having just an idea won't get you anywhere.. you need to find a passion and vision and then, and only then, will you have a great website. I'd suggest thinking about the needs of users/people and see if you can conjure up something from there.
My only tip for you is NOT to try and compete with larges sites like YouTube. Just because they're successful, doesn't mean you will be. Try to be innovative.
a site that users can upload stupid and pointless videos... o wait... its called youtube... DAM lol
A tech website...but focus it on one area (maybe web design)