When investing in company (with stock purchases or whatnot), you'll have to pay attention to their business operations & projections so you have a rough idea on when you should buy (generally in anticipation of any good news or beneficial information for the company... like product announcements or releases) to make money as the stock price will likely rise as a result.
If there's bad news for a company, you'll either want to sell your stock OR wait to buy stock since there's a likelihood of the stock price dropping. Advanced traders may attempt a Short-sell (where you're immediately selling borrowed stocks at higher value & repurchasing the borrowed stock at a lower value), but it can be a very risky tactic.
If you're going for the long-haul (holding a stock for months or years) instead of day-trading (attempting to make a quick buck), buying stock in a healthy company like Apple, Alphabet or Berkshire Hathaway is typically good investment. HOWEVER, if the company's health is poor or failing like Sears Holdings, you could potentially lose your entire investment.
While it's your choice on how much & what companies you wish to invest in, I hope this sheds light on the subject & that you do a little more research into your desired companies.