How do I acquire a potion for my trunk?

You have to make it using the potion system, which you can find out everything about here - viewtopic.php?t=22602

Once you've made the potion, it will be added to the trunk.

Quick clarification here (just to be sure there aren't any misunderstandings):

If you're asking about your encyclopedia Trunk, the answer is the same as for learning spells: You can do homework (mostly Potions class) and upon receiving a grade better than T, fill out the Encyclopedia Update Form to get the potion added to your Trunk by the staff team. This is equivalent to your character having the knowledge how to brew the potion, it does not mean your character has it available and can use it.

To use a potion, you need to have it in your inventory trunk (where your other items are, you can find it in the drop-down menu below your character picture on the top right). For this, as Nora said, you need to brew it using the system. You can only brew potions that you know how to brew (i.e. have in your encyclopedia Trunk). Once you've done that, you will receive a potion item from the staff team. This is equivalent to your character having the brewed potion available for them to use.

Thank you very much for the info!