There are 2 key points to remember:
Ultimately the price of shares is based on demand, which in turn depends on investors' perception of the company's future prospects. If other investors feel pessimistic about a company, the share price may fall. In extreme cases the company could go under - and you could lose all of your investment.
Some shares are riskier than others. You should carefully consider your appetite for risk, and select shares you are comfortable with based on this.