Generally there are three main reasons to sell your stocks and shares. The target price you specified when investing in a particular stock or share has been reached. Major changes in a company that you are investing in are occurring. Lastly, a rapid rise in a stock or share value is in process.

Each of the above listed bullet points should be taken individually in slightly more detail. Many people invest in stocks and shares for retirement purposes, that is to say, you or someone on your behalf invests for a term that ends coinciding with your retirement age. Hopefully at that time your investments have performed well and you get more than you initially paid in, however, this is not always the case. Usually investments for retirement are spread over many companies so, in theory, they won't all perform poorly. If you are lucky they will all perform very well so you can spend your retirement securely.

Secondly, if there any major changes in a company that you are investing in, or major changes in the industry that a company operates in, this could cause rapid fluctuations of the values of stocks and shares. Sometimes, however, one has to hold ones nerve and ride out the storm as there is a chance that the value will go back up, you may choose, however, not to gamble and sell your investment. The rise and fall of the stock market is normal behavior. You can find many stock brokers on the Internet, some of which will allow you to "play" the market with fictitious money with live stocks and shares, you can the gauge to a degree whether its for you or not before investing your own real cash.
 
A rapid rise in a stock or share brings us to the third in the list of reasons to sell. This is an area that excites many investors of the stock market. They watch for these sudden peaks of activity then sell. If they get their timing right vast sums of money can be made, by getting it wrong, however, vast sums can also be lost.
 
If you are a beginner to the stock market then you would be better working with a stock broker or your financial adviser. There are many to choose from online so research them thoroughly. Investing small sums of money initially to "play" the market can be fun then, if you are clever and with luck you will reap the rewards.