Stock mutual funds or exchange-traded funds. These mutual funds let you purchase small pieces of many different stocks in a single transaction. Index funds and ETFs are a kind of mutual fund that track an index; for example, a Standard & Poor’s 500 fund replicates that index by buying the stock of the companies in it. When you invest in a fund, you also own small pieces of each of those companies. You can put several funds together to build a diversified portfolio. Note that stock mutual funds are also sometimes called equity mutual funds.
Discount brokers used to be the exception, but now they're the norm. Discount online brokers give you tools to select and place your own transactions, and many of them also offer a set-it-and-forget-it robo-advisory service too. As the space of financial services has progressed in the 21st century, online brokers have added more features including educational materials on their sites and mobile apps.

Risk tolerance is a psychological trait that is genetically based, but positively influenced by education, income, and wealth (as these increase, risk tolerance appears to increase slightly) and negatively by age (as one gets older, risk tolerance decreases). Your risk tolerance is how you feel about risk and the degree of anxiety you feel when risk is present. In psychological terms, risk tolerance is defined as “the extent to which a person chooses to risk experiencing a less favorable outcome in the pursuit of a more favorable outcome.” In other words, would you risk $100 to win $1,000? Or $1,000 to win $1,000? All humans vary in their risk tolerance, and there is no “right” balance.

The solution to both is investing in stock index funds and ETFs. While mutual funds might require a $1,000 minimum or more, index fund minimums tend to be lower (and ETFs are purchased for a share price that could be lower still). Two brokers, Fidelity and Charles Schwab, offer index funds with no minimum at all. Index funds also cure the diversification issue because they hold many different stocks within a single fund.
It is also important to know what you want to accomplish with your investments before you actually invest. For example, you might want to purchase a home, fund a child’s college education, or build an adequate retirement nest egg. If you set financial goals at the outset—and match your investments to achieve those goals—you are more likely to reach them.
When it comes to investing for long-term growth and putting your money to work, it is immensely important to understand your goals and the investment philosophy you will adhere to. It can be easy to lose sight of your targets amidst the noise on social media or news outlets surrounding the latest and greatest investment trends, but if you define your goals and investment strategy, you can stay on track.

A stock's market capitalization (cap) is its true value, the sum of the total shares multiplied by price. It has more meaning than the share price because it allows you to evaluate a company in the context of others of the same size in its industry. You can use a market cap as a filter to screen for companies to balance your portfolio. A small-cap company with stock capitalization of $250 million to $2 billion shouldn't be compared to a large cap, which ranges from $10 billion to $100 billion. Market capitalization influences your investment returns. 
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