Don't know how to decide which stock to buy.
Don't know when to get out of a stock that is losing money.
They don't know when to sell a winning stock.
They don't know how to make a good portfolio.
Don't know how to pick the right stock to buy?
How do people who have never bought shares before decide which ones to buy? You could listen to your stock broker, your "experienced" relative, or a free "share pick" on the internet, etc., but you will lose money.
Because each share's behaviour is so complicated, only the most professional full-time traders have the right tools to make good decisions about which shares to buy. This kind of experience and technology is just not available, especially to traders who are just starting out.
Don't know when to get out of a stock that is losing money...
Beginner traders are most likely to die because they don't know when to get out of a losing share. Too many traders keep their stocks until they are worthless. Most beginners will hold on to the stock in the hopes that it will go up again, because there is no way to know if a stock will ever go up again. A beginner can only keep from losing everything if an expert tells them when to stop trading.
You don't know when to sell a winning stock...
How many times have you heard of people holding on to shares that made them a lot of money until one day, the share turned against them and caused them to lose a lot of money?
Too many people think that their winning stocks will keep going up forever, so they never know when to take their money and run... until their stocks crash. Again, the problem is that it is very hard to tell when a stock is losing its upward momentum.
You don't know how to put together a good portfolio...
Do you know that a lot of stocks go up and down at the same time no matter what? Do you know that some stocks move in the exact opposite direction of other stocks? Do you know that many stocks move in the exact opposite direction of the market? Do you know there are some stocks that never change? Do you know that some shares are about to be taken off the market?
How could you ever put together different shares in a smart way to make money if you don't know the above? What happens if you put a share with one that moves in the opposite direction together? Will you ever get paid?
Because of this, a lot of people are trading Market Index or Market Index ETF, which are much more stable and reliable.
At http://www.mastersoequity.com/MOE ridetheflow.htm, you can read about how trading an index-based strategy can help you deal with all of these issues.
