Migrate Server 2003 to Server 2012 R2

I am currently trying to migrate active directory, dns and file roles over from a 2003 server to a 2012 r2 server. I have installed the Windows Server Migration tools but from there, there isn't much guidance on what to do. How would I transfer AD from the old server to the new server? Also the same for DNS. I can just recreate the DHCP scope as it isn't very complex.

Thanks!