Make sure you use all your tax vehicles to their extent.
Make sure you use all employee matching programs to their extent.
With stocks, almost no one out picks an index fund over time.
Be mindful of your costs (especially with funds). Hidden fees can eat up any gains you make.
Never mistake a bull market for intelligence.
Definitely the best advice on this thread.
The best way to learn is to have some skin in the game, but just be reasonable with allocations. Ideally, don't invest more than 20% in an individual stock. Also, buy stocks in different industries (e.g. Bristol Meyers, Starbucks, Duke Energy, Apple, and FedEx).
Do NOT buy all at once. If you have $10,000 that you would like to put in the markets, buy one or two stocks/ETFs in $1,000 - $1,5000 increments so that you can achieve different price points (cost basis).
Use limit and stop orders rather than a market order to buy and sell. A market order will simply buy a stock at its current price, which is okay, but unless you are a day trader, you can likely afford to be patient for a better price. So you can use a limit order to buy a stock like Wells Fargo (currently at $46.35) and set a lower price point ($44.00) with a 2 month horizon. If WFC ever drops to $44.00 over the next 2 months, then the order will be automatically executed. Additionally, you can set stop orders to sell a stock if it drops 10% (or whatever % you like) so that you don't have to worry about constantly monitoring your portfolio and you can preserve the majority of your capital in the event of a stock market crash.
If you have a longer term investing horizon (aka not retiring soon) then reinvest all of your dividends. As others have mentioned, dividends are awesome. Reinvesting dividends is not only tax friendly but it compounds so you will receive more and more dividends in future quarters in addition to increasing gains on an appreciating stock. Not every stock allows for dividends to be reinvested, but most do.
For potential investment ideas, look at some of the brands you currently use, companies headquartered close by and brands you see your kids/kids' friends supporting. Use this as a starting point only, and then look at income statements to see if those companies are increasing their Net Income & Revenues year over year. While Jim Cramer is not God, Mad Money on CNBC can be informative and try watching that if you can tolerate him to get additional investment ideas.
For me, investing is the perfect combination of fantasy football and betting. Everyone can be successful at it, if they take it as seriously as their fantasy team. Do not simply buy and hold, that would be like drafting a team and never setting the lineup. Every quarter, 6 months, or year critically evaluate your portfolio and determine if you need to make any changes. Do not be afraid to sell a stock that has appreciated significantly. Maybe one stock has a high beta (meaning that it's more volatile than the overall market which always has a beta of 1) and you would prefer something less volatile.