New user email notification not being sent
Install and use the SMTP module: https://drupal.org/project/smtp - Period. Nothing comes close to using SMTP and you'll avoid current and future headaches. Details are below. Good luck!

Why sending via the server is bad:

The post recommended by Anil Sagar is good. However, based on my expertise with web hosts, especially DreamHost, you want to avoid at all costs sending emails from your server.

By default, Drupal sends email via the server it's residing on. That server might not have email configured. That server might have email configured now, not properly configured in the future. That server might get spam listed in the future. In summary, a lot of things might happen that leave you without email sending capabilities, and most probably you will not be notified of the issue.

Also, you're lucky that your email is "not working" as, in most cases, some emails via the server will be sent, others will not, which is even worse as you'll not even know you have a problem to fix.

Why sending via SMTP rocks:

When you send via SMTP, you're relying on an external dedicated mail server. Choose a solid mail server provider (Gmail, Yahoo, Zoho, etc) and you'll be on the safe side. They handle maintenance and everything for you and all you have to worry about is the first step which is setting up and installing the SMTP module.

Note about SMTP sending limits:

If you're sending a large number of emails, most good email providers will have a daily limit that you'll reach and they'll stop sending your emails. This is a way for them to fight spam. For Gmail, I think the daily limit is 250 emails, which is plenty for most needs. If you have larger volumes, you might want to buy a paid SMTP account with a large limit or consider something like: http://www.mandrill.com/ - You can send up to 12,000 per month for free. They also have a Drupal integration module: https://drupal.org/project/mandrill

And if you're sending a large number of emails via your own server, you're ending up on a spam list from day one and your emails will soon not even reach junk mail.