How to choose your websites domain name:

1. It should be something that goes with your website topic.
2. You should make it short, if you can’t then but some hypens-to-make-it-understandable
3. Make it something understandable. Is your visitors going to remember it ?
4. Make it clear, most of the times affliate programs such as Adsense displays ads according to your domain name and content.

Key Point: If your website is about hamsters, I recommend you that you don’t get hamsters.com instead get what its called brandable domain just like syrian-hamster.com

What extensions should I use ?

Really it does not matter.  Whatever you are comfortable with is just fine because the search engine don’t care about the extensions most of the time.  But DOT COM domains are much more likely to be remembered by the returning visitors of your site, besides everybody likes DOT COM domains better.