From what I recall, he seems to have an ability to shield himself from Xavier when Xavier was mentally chatting with Eric. Then when Apocalypse interferes, he seems to connect to Xavier's mind and use Cerebro to launch all nukes from Earth.
So is Apocalypse a telepath and if so, why does he need Xavier at all?
My guess is Xavier's mind is still young and could be more powerful than Apocalypse's but perhaps this was explained (I only saw the movie once).
Traditionally, Apocalypse has displayed various amounts of telepathy in the comic books, although he doesn't seem to be especially skilled in it in the film:
He does, as you say, display an ability to shield himself from Xavier's telepathic abilities.
Regardless of his exact level of telepathy, Apocalypse explains exactly why he needs Xavier: Charles can "be everyone". With Xavier's power, Apocalypse can inhabit the minds of many individuals all at once and control their actions, giving him simultaneously the power of any mutant and rendering him virtually immune to the kind of revolt that entombed him during his previous reign.
Comic versions of the character indicate some degree of telepathic ability but nowhere near the class of most experienced telepaths. His mind is powerful enough to prevent them from entering, reading or altering his mind in any way.
In X-men: Apocalypse, his mind is able to sense emotional states indicating an empathic awareness of the minds of others. He also exhibited the ability to control and manipulate technology, a form of mecha-kinesis. This is how he learned languages instantly once he had access to Storm's television.
His superior intellect allowed him to learn languages nearly instantaneously once exposed to the mechanized signal from the television set. It is this same unique combination of incredible intellect, mutant mind, and centuries of experience which made any form of telepathic interaction with him dangerous FOR the telepath foolish enough to use their powers on him.
In the case of Xavier and Cerebro, he doesn't take over Xavier's mind, he uses his mecha-telepathy and his control of machines to use Cerebro to dominate Xavier's will and through the machine, he uses Xavier's mental powers to launch the weapons.
Once he experienced true telepathic ability through his interaction with Xavier, he was able to essentially hijack the mind of the telepath, overcoming their psychic will with his more powerful mutant mind.
Xavier, believing his mental powers would make him capable of confronting Apocalypse in the arena of the mind, makes the mistake of confronting Apocalypse with his mental abilities. This was a mistake on Xavier's part. While his powers were formidable he didn't understand what centuries of mental discipline would do buttress Apocalypses' mind.
Xavier is unaware of Apocalypse's experience with protecting his own mind. In addition, Apocalypse has been shifting his mind into the bodies of other mutants for centuries, dominating and destroying their will once he has taken over their bodies.
Apocalypse has centuries of extreme mental discipline, moving his mind from body to body, absorbing mutant powers for generations, and his mental fortitude is revealed when he returns to life after thousands of years of life with his mind intact.
Thus when Xavier confronts him in the mental plane, Apocalypse quickly adapts and takes control of the battle, reorienting himself making the struggle similar to any other domination battle he has engaged. Xavier never had a chance.
Once Apocalypse took Xavier's powers, he would have OPERANT mental powers instead of his passive mental defenses. (Operant mental powers would be any powers that were not passive and only available when acted upon, commonly called mental defenses in the Marvel Universe.) Apocalypse would instead have the ability, through the strength of his formidable mind and Xavier's powers, to instantly dominate every will on Earth.
While Xavier could likely take over the minds of everyone on Earth for a time, his human stamina would not allow him to maintain this state for long. With Apocalyspe's near limitless physical vitality he would be able to read the minds and enslave the entire Human race in an afternoon, altering their psyches permanently.
