You have been testing and now ready to go live. Maybe you have a single or two tier solution and now you need to move to a traditional three tier solution. I know there are several options and many ways to migrate but simply stated you have two major options. New farm or use the existing farm. Here are some high level ideas to help you out. This will be a large project so testing will be key.
If you create a new farm you will then need to migrate your databases over. There are several third party tools or you can use powershell to assist. The advantage in building a new farm is that you start with a brand new farm and can recreate the farm in the knowledge from your testing environment. However testing is key to make sure the old farm and the new farm are the same.
If you are going to use your existing farm, the advantage is that this farm is exactly the same you are just adding / moving / removing servers as needed. This article is a good start and gives good overview from Microsoft on Multiple servers for a three-tier farm (SharePoint Server 2010)
http://technet.microsoft.com/en-us/library/ee805948.aspx
Here is another good article that give a step by step with screenshots
http://sharepointsolutions.com/sharepoint-help/blog/2011/02/how-to-scale-out-a-sharepoint-2010-farm-from-two-tier-to-three-tier-by-adding-a-dedicated-application-server/
Move All Databases
http://technet.microsoft.com/en-us/library/cc512725.aspx
Move Content Databases
http://technet.microsoft.com/en-us/library/cc262792.aspx
How To- Move Content from old 2010 farm to new 2010 farm in-use without triggering Alerts (and without stopping SMTP)
http://social.technet.microsoft.com/Forums/en/sharepoint2010setup/thread/fbe979b3-a7d4-4963-9cac-ac74e246f6f4
Moving a site from one Farm to Another Farm sharepoint 2010
http://social.msdn.microsoft.com/Forums/en/sharepoint2010general/thread/5418151e-ad60-40de-93be-9fbb359e371e
How to move a SharePoint standalone 2010 farm from a server to another?
http://social.technet.microsoft.com/Forums/en-US/sharepoint2010setup/thread/37aae074-99cc-4253-a387-a1245ff68910
OLD - Issues faced when moving SharePoint to a new domain
Moving SharePoint to a new active directory domain
http://justgeeks.blogspot.com/2007/09/moving-sharepoint-to-new-active.html
move SharePoint to a new domain
http://sharepointapplied.com/2009/02/04/move-sharepoint-to-a-new-domain/
Reference
http://sharepoint.stackexchange.com/questions/26769/move-sharepoint-to-new-domain