Do we have voluntary control over what we believe?
In general, no. The credence we place in various propositions--our belief or rejection of them--is largely out of our voluntary control and dependent upon our perceptual experiences, memories, other beliefs, and established habits and methods of belief formation and revision. We can indirectly cause our beliefs to change by engaging in actions which change our habits--seeking out contrary information, learning new methods like forms of mathematics and logic, scientific methods, reading books, listening to others, etc.
How does someone become a skeptic?
People aren't born as skeptics--they learn about skepticism and how it has been applied in various cases (only after learning a whole lot of other things that are necessary preconditions--like language and reasoning). If skepticism coheres with their other beliefs, established habits and methods of belief formation and revision, and/or they are persuaded by arguments in favor of it, either self-generated or from external sources, they accept it and, to some degree or another, apply it subsequently.
When someone becomes a skeptic, what happens to all of the other beliefs they already have?
They are initially retained, but may be revised and rejected as they are examined through the application of skeptical methods and other retained habits and methods of belief formation and revision. Levels of trust in some sources will likely be reduced, either within particular domains or in general, if they are discovered to be unreliable. It's probably not possible to start from a clean slate, as Descartes tried to do in his Meditations.
Is everything a skeptic believes something which is a conclusion reached by scientific methods?
No. Much of what we believe, we believe on the basis of testimony from other people who we trust, including our knowledge of our own names and date and place of birth, parts of our childhood history, the history of our communities and culture, and knowledge of places we haven't visited. We also have various beliefs that are not scientifically testable, such as that there is an external world that persists independently of our experience of it, that there are other minds having experiences, that certain experiences and outcomes are intrinsically or instrumentally valuable, that the future will continue to resemble the past in various predictable ways, etc. If you did believe that skeptics should only believe conclusions which are reached by scientific methods, that would be a belief that is not reached by scientific methods.