Buying stock to hold, but then what

I am interested in holding a few stocks just 'for fun.' More like to gain experience and learn about the stock market. Here's my question:

I have read if you sell a stock within the 12 months since purchase, you are liable for 50% tax on your returns. So, if I buy in June an 'x' amount, what do you do if the stock goes down in 6 or 10 months in? Sell it? Or do you just continuing buying more when it goes down? I know it will fluctuate all year, but say it never goes past the amount you had initially purchased. How quickly should you get out? Or is that the risky practice of stock trading I guess. The whole point is holding the stock a whole 12 months to avoid that 50% tax on returns.