Let me explain about the magic system:
You need people to believe you can do X.
Difficult feat needs more believers.
There's no penalty from those who disbelieve what you can do.
Your power increase linearly with your believers.
The protagonist just read an ancient version of "The Secret". This book teaches that instead of you believing you can do something, you need other people to believe you can do something.
What a revelation! In the book, there are many examples: you can throw fireballs. You can become invisible. You can even resurrect the dead if only there are enough people believe you can do that! One catch (or two, actually)... you need to know this secret, and you can't actually know how many people needed to perform something.
Convinced, this soon-to-be mage stepped out to embark his journey becoming a world-class wizard. But he doesn't even have a single believer yet!
How this young man convince people to believe him to start his career, if he doesn't even have a single believer yet?
This story is set on our modern world, and the Secret is already lost, so right now he is the only one know about the Secret.
Basically the system works like an illusion; realization about the truth will dispel the illusion. If a believer is convinced that this young man is just a normal human who cannot do extraordinary things, he will lose the power from this believer (unless he start believing again).
Note: This is not "what my character should do" question, but rather "how to achieve this, given the system and the problem (no believer to start with)".
Update: Manipulating a person is difficult, although not impossible, because it conflicting with the person belief. If the target person has weak beliefs (in other word willpower), it will be easier.
Manipulating a person into your believer is possible, but it will permanently depend on the other believers. If the belief that "he has manipulated X thought" fades, the effect also gone. Thus creating believers by manipulating thoughts is like an attempt to create a perpetual machine.