You can use all your daily powers once per day.
You can use all your encounter powers once per encounter.
You can use all your at-will powers at-will.
If you're wondering how many powers a player gets, the player gets to pick one power of each level they qualify for.
For example, a level 1 player gets any racial and/or class powers(channel divinity, defender at-wills, cantrips, fey step, elven accuracy, etc...) plus 2 at-wills plus 1 encounter power plus 1 daily.
At level 2, the player gets to pick one utility power from the level 2 utilities.
At level 3, the player gets to pick an additional encounter power from the level 3 or level 1 encounter powers.
As far as magic items, you can use 1 magic item daily per day, +1 per milestone reached.
Hope that answers your question!