301 redirects are used to tell the browser that the page they are requesting has been moved permanently to another location. However, this is often used in SEO as well to give a domain ALL the credit from incoming links.

Search engines see your domain as 2 domains.

http://www.mydomain.com

and

http://mydomain.com

If 50 sites link to www. and 50 link to mydomain.com, you are losing half the 'votes' for your website (a link to your site is essentially a vote that your website has good content). The best way to get FULL credit for your links is to setup a 301 redirect.

Log into your hosts control panel and look for a Redirects option.
Type of Redirect: 301 (Moved Permanently)
Select the domain from the dropdown list.
WWW Redirection: Look for an option that says Only redirect with www.
Redirect to: Type http://mydomain.com

Click add.
If you visit http://www.mydomain.com, you should now be redirected to http://mydomain.com. Alternatively you can set this up so that any request to http://mydomain.com redirect to http://www.mydomain.com, this is up to you.

One last thing use this: SEO Friendly Redirect Checker to make sure your redirect is setup properly! For Example, i can check this site for a redirect by typing http://www.theinfiniteblog.com, it will tell you there is a search engine friendly redirect setup!

If you do not have a host or they do not support configuring redirects, you may have a little more trouble setting up the redirect, BUT, there is a great resource for setting up 301 redirects with: IIS, HTAccess, PHP, ASP, etc etc.
How to Redirect a Web Page

Happy Redirecting!
Comment if this helped!