How do the prices in a market change?

Markets are guided by the forces of supply and demand. Every time a user buys a share in a particular idea or outcome, that demand forces the price up a little. The more people buy, the higher the price becomes. If the price gets too high, people will do the natural, self-interested thing and sell their shares to get a profit (or sell shares that they don't own on credit). Similarly, each time a user sells a share, the price goes back down some.