If you are using Magento, this free plugin will auto enable GTM with the dataLayer values. The GTM-account ID is set within the configuration.
Note: a separate GTM-xxxx container id can be set within Magento’s store view level for dev.clientdomain.com and www.clientdomain.com see details here. This plugin does not yet support enhanced ecommerce or onclick one-page-checkout tracking here, and a JSmacro patch is needed for previousVisitorLifetimeValue calculation here.
Also Fooman has added GTM dataLayer support, but it`s still in beta here & W3C magento plugin is here.