Since the underlying businesses operate in differing markets, sectors and countries, their quoted prices move independently as supply and demand in them rises and falls and new information is released to the public about the current business situation. It is the changing of prices that offer investors the opportunity to make a capital gain (or loss) via ownership.
There are plenty of interesting and simple online tools which can be used to improve your trading results, such as the Relative Strength Index (RSI). This is just one example from among dozens of possible technical analysis (TA) options, and you will need to discover and decide which ones work best for you. You may need to paper trade to figure out the best TA tools for you and your strategy. Besides these sorts of technical analysis indicators, there are a few "tried and true" rules:
Before making your first investment, take the time to learn the basics about the stock market and the individual securities composing the market. There is an old adage: It is not a stock market, but a market of stocks. Unless you are purchasing an exchange traded fund (ETF), your focus will be upon individual securities, rather than the market as a whole. There are few times when every stock moves in the same direction; even when the averages fall by 100 points or more, the securities of some companies will go higher in price.
Should the company management and majority owners choose, they can pay one or more dividends per year to stockholders. The money for these dividends will typically come from profits earned within the business. In most countries, these dividends are subject to income tax payable by the receiver. Often there is a withholding tax taken at source to ensure that non-resident shareholders pay as well. 

News events and earnings reports can change the perceived value of a company. Because the stock market functions as an auction, prices sometimes need to adapt for a trade to occur. When there are more sellers than buyers, the price will go down. Alternately, a stock that has more who want to buy than sell will experience a price increase. Buyers and sellers can be individuals, corporations, asset management companies, or others. Price fluctuations can be dramatic in just one day.
Successful traders have to move fast, but they don't have to think fast. Why? Because they've developed a trading strategy in advance, along with the discipline to stick to that strategy. It is important to follow your formula closely rather than try to chase profits. Don't let your emotions get the best of you and abandon your strategy. There's a mantra among day traders: "Plan your trade and trade your plan."

What would make me sell: Sometimes there are good reasons to split up. For this part of your journal, compose an investing prenup that spells out what would drive you to sell the stock. We’re not talking about stock price movement, especially not short term, but fundamental changes to the business that affect its ability to grow over the long term. Some examples: The company loses a major customer, the CEO’s successor starts taking the business in a different direction, a major viable competitor emerges, or your investing thesis doesn’t pan out after a reasonable period of time.