These days, I see many people investing in companies without even knowing what it does. 
If you are a trader that analyses charts, follows momentum, or look at other technical factors, sure. But for the majority of us, investing in what we don't know will lead to disaster.
"Stick to your circle of competence" - Warren Buffett
The worst is, these people are influenced by emotions and panic sell when they see a red day.
What happens when you know what you're doing, and what you're investing in?
You won't be as affected emotionally and can make rational decisions. Since you know and believe in what you are investing in, emotions won't get the better of you when you see huge fluctuations in stock prices.
These are the basics to selecting companies:
1. Understand what the company does(your circle of competence)
2. How does the company make money and can they keep growing long term?
3. Use valuation methods such as P/E ratio, DCF, P/S ratio etc. to estimate the intrinsic value of the company
4. Is management of the company good, or ridden with scandals?
5. Are you comfortable holding this stock long term even if it goes down?
Being patient can be tough sometimes. But think about it this way - you don't have to do much once you decided on a company. If you done your research and fundamental analysis correctly, you don't have to worry about it.
It is only when something has fundamentally changed about a company that you need to evaluate it again.
Good luck!