Www in url

This may be a kind of dumb question, but I’d read somewhere that you can make it to where your website will always have www in url.

for example: http://myaddress.com = http://www.myaddress.com

I’d tried searching to find out how to do this but I’ve been unable to find out. My site has users log in and if they log in at http://myaddress.com their session doesn’t carry over to http://www.myaddress.com so I need my url to always include the www.

Thanks!! :red: