Provoke then shield lob/tomahawk. Since you are still new to tanking there maybe a bit of hesitation on your part. You need to target the add as soon as you see its hp bar on the enemy list. If you're on PS3 this will always appear first before the actual mob appears on screen depending on your connection.

For example: In T4 at the start of the second phase when the soliders/knights drop, I see their hp on the enemy list first so I spam flash and when they appear a second later on my screen they were registered as being flashed and thus I have established aggro.

Being new to tanking the one piece of advice I can give you is to be adaptable and always know when and where the next adds/boss are going to be and where they should be so the rest of the party can do their job. There may be times where you might have to take over when the MT falls (especially in Titan EX) and that's where you're true test of tanking begins. Good Luck.