I would like to know about the stock market?

When you buy a stock (for example at $50) you do not realize the profit or loss until you sell it. If during the time period you are holding the stock, it falls down to $40 then goes up to $60, you do not actually lose $10, then earn $10, unless you sell it. The profit or loss are shown on your account but are not realized (called "paper" loss or profit). This is true only if you buy and hold stocks. When you are invested in a stock, the invested money do not earn any interest, it just fluctuates with the stock price. The only instance when you make additional money is when/if the company distributes dividends while you are holding their stock.
This is not true if you are buying stocks on margin or if you are trading on derivartives (these are risky if you do not know what they mean). And as a recommendation, never invest in something you do not fully understand.

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Say you buy the stock for $50. You spent $50 to do that if it goes down to $30 you now only have $30. If it goes up to $70 you now have $70.
Easy as that.

No, you own the stock you don't pay more because it lost it's value. If you don't sell it at loss then you still own the stock and it could have future gains. Make sense???
Why not a do a little reading at marketstockdotnet for more clarification

if you only have one stock and you use 100% margin and the stock drops over 50% the next day, then you have to pay in more money.