Well, visualisation or focused thinking or whatever you want to call it can have some influence on your own self (not will have, mind you, but can have). Placebos are a good example of that, as are hypochondriacs. But mainly, that’s either changing how you think you feel, or else you managing to trick your body into releasing more or less hormones. Making your body sprout tumours is a bit different from that.
Positive thinking can also have an important part to play in shaping the world around you. But not by simply thinking “Wouldn’t it be nice if my house was all repainted?” and then sitting back to wait for it to happen by the power of thought. Rather, it’s the first step in you actually getting up off your backside, going to the store, buying paint and brushes, and then getting to work on your walls. In that way, at least, thinking about something can result in it happening.
Whichever case, it’s not any sort of a great “secret” or “process”.