Good words of advice from Eddie already. Sounds like you will be there for prime time. All I can add is to learn how to calm your nerves - your first sight of a 150lb fish can do strange things to the neurons and synapses
Keep practicing like your doing. The 12wt will start to feel like a friendly old jacket after a while.
When Tarpon hit I like to make several hard strip-strikes. Other anglers have different techniques. What is important is learning how to put maximum pressure on the fish. Whilst you're practicing, rig up a few leaders and tie one end to something solid. Wear glasses! Now back up and feel how much weight you can put on the leader. Really lean into it. Build it up 'till it breaks, then do it again, and again and again. You'll find that you have to keep the rod fairly straight, otherwise it will break before the tippet. Max. prssure is good for the fish and good for the angler
Have a blast, take pics and let us know how it went. Ohh, and welcome to the forum