Redirect how to?

[ComputerVitals]

When someone comes to my site on computervitals.com
I need them redirected to www.computervitals.com


If they log in on computervitals.com and click login, it will log them in and redirect them back to computervitals.com and not be logged in. But if you then put the www. in front you will be logged in.

I think there is a way to do this via htaccess but I dont know how.

[ComputerVitals]

I think I have it.
seem like all my urls are now being redirected.

I used

RewriteEngine on
rewritecond %{http_host} ^computervitals.com          [nc]
rewriterule ^(.*)$ http://www.computervitals.com/$1   [r=301,nc]

rewritecond %{http_host} ^computervitals.net          [nc]
rewriterule ^(.*)$ http://www.computervitals.com/$1   [r=301,nc]

rewritecond %{http_host} ^www.computervitals.net          [nc]
rewriterule ^(.*)$ http://www.computervitals.com/$1   [r=301,nc]