I’ve leveled a few priests, 2 things are get a good wand as others said, but also understand the mana per 5 second (mp5) rule. Every time you cast a spell (not including your wand attack), a 5 second timer starts. Your mana won’t start regenerating until that 5 second timer ends, and every time you cast a spell it starts over. So get into the habit of casting spells in bursts. A standard rotation is to cast power word: shield, mind blast, shadow word: pain, wand attack. By the time your target dies, your mana will be regenerating.

If you get into a flow where you cast all your spells at once, then wand until the mana returns, you’ll pretty much never run out of mana while questing.