Migrating SQL Server 2008 databases to new 2-node WSFC SQL2014 Alwayson AG
To perform a server migration to an AG using log shipping, you should log ship to both servers. When you are ready to actually cut over to the new AG, follow the following steps:


Take a final tail log backup on the old instance (BACKUP DATABASE...WITH NORECOVERY)
Restore the backup to both AG nodes (or allow your log shipping to apply it) WITH NO RECOVERY.
On Node 1, bring the the database online (RESTORE DATABASE MyDb WITH RECOVERY)
Add the DB to the AG on Node 1.
Join the secondary DB to the AG.


I'd recommend practicing this routine on your QA system before the real production upgrade. There are a lot of scripts/statements to be run, and lots of switching between servers to do it. It's best to have those steps carefully documented and scripts staged in advance.