The longer your time horizon, the more you can afford to invest in stocks. When you're investing for retirement ten or more years away, you should have substantially all of your assets in stocks. Practically speaking, most people invest for more than one purpose, so if you pick a group of funds for each goal, the overall result will be a portfolio somewhat balanced between stocks and bonds. Second, honestly assess your tolerance for risk and volatility. You must be able to withstand the stock market's downturns to benefit from its long-term generosity.
People tend to view the stock market as a monolith, with everything moving in lockstep.
Instead, try thinking of it as a bunch of markets. There is a stock market for big companies, which behaves differently from that for small companies. By the same token, stocks of other countries don't move to exactly the same beat as those in this nation. Trends could favor European markets over those in Asia, or Japanese stocks over those in Korea, and so forth. So first decide how much of your stock investments should be in U.S. stock funds and how much in funds specializing in foreign stocks.