When choosing where to trade, do not rely on any site that can't point to a 100-percent unbiased guarantee. Regardless of what they call it, you only want to trust a website or service that ensures your best interests are front and center. They should commit to everyone that they will not trade in the shares they tell their customers about, and that they're not simply touting their own investments.
It can be helpful to start with paper trading, or simulated trading that allows you to practice without risking actual money. By keeping track of pretend money, and making imaginary trades, you'll learn what tactics work and what sorts of penny stocks provide you with the greatest profits. If you lose on your trades, you don't lose cash in real life, and ideally, you'll learn some things that you might be doing wrong.
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.
One constant principle of investing is that markets fluctuate. Stock prices will rise and fall for a number of reasons: the economy, investor sentiment, political uncertainty at home or abroad, energy or weather problems, or even corporate scandals. This means market performance isn’t always predictable. That is why diversification, or spreading the investments in your portfolio among different asset classes and across different sectors within each class, is such an important strategy. Diversification is a time-tested way to manage risk.
Bonus Stock Market Tip: Everything above is related to how best to invest actively - in other words buying and selling into companies that have been selected by you. But what if you don't have the time, money or inclination? What if the paragraphs above put you off? Perhaps you were looking for a simpler guide? The stock market for dummies perhaps?