What is one thing I should know before I start investing?

For me, I think it would be understanding what you are investing in as much as possible. I really think that it is very helpful when you know the inner workings of the investments and therefore can make better predictions and forecasts.

Another I think which would be quite important is also being emotionally calm, both during bullish and bearish runs. Following the herd may not be a very good idea since much of it is driven by market sentiment, which has an emotional component also. However, if you know the fundamentals of your investments, and can understand the market conditions, then i would suppose you might have a much easier time trying to make decisions during volatile periods.

The 8th wonder of the world: compounding. This effect of allowing you to earn returns on top of your capital gains is what makes investment so attractive, and so exponential in terms of allowing you to grow your wealth. Thats why you should always start early, so that you can take advatange of time, as time is your friend with regards to compounding. If you invested $1000, at an average annualized rate of 8% for 30 years you will receive $10062.27 - a 10 fold increase in your wealth. Imagine 10 000, or even 100000!

But there is also costs, and with regards to costs, time is not your friend, but your enemy. If you do not make sure to check the costs of your funds or ETFs that you may buy into, you may incur substantial costs that over time eat away the potential full gains of your investments. If the costs of your fund averaged at 2% a year, meaning your average annualized rate is 6%, your $1000 gives you $5743.49, barely 5.5 fold. You lose almost half your potential gains from just a 2% cost!

The numbers don't lie - compounding is both a wonder, and a potential curse. Always make sure that you check the costs of any ETFs or funds you buy into, and make sure they are reasonable for their service - the money you don't get is money you lose!