First you choose a specialty, appropriate to psionic tropes. Telepathy and telekinesis comes to mind. We should resist the urge to create a psionic specialty equivalent to every school of magic, as has been done before.
You get some simple at-will stuff for your specialty. Telekinetics can push and trip people at a distance and lift objects with their brains. Psychics can charm people, attempt to read their surface thoughts, and even cause small amounts of psychic damage.
Then you have power points, which recover slowly after you use them (essentially making them encounter resources). You can use them to augment your specialty, so that a powerful enough telekinetic could lift someone and toss them into an ally, dealing damage to both. A powerful psychic could crush an opponent's will and make them into a thrall, or overwrite someone's personality with a copy of his own.
So basically it's an encounter resource system modifying at-wills which are granted according to your psionic specialty.