If I had to pick a number one rule in trading, this is certainly a good candidate. It might not be the most important trading rule, but it is a good first one. Try not to get way down, money-wise, right from the outset when starting out trading (can also be when you return from a vacation).
It is not fun if you have to spend days digging yourself out of a hole you got yourself into in the early days of trading.
Start slow, observe and practice on a demo account. Give yourself time to watch the flow of the market and see how it reacts to news and reactions in order for you to get yourself into the feel of it and the rhythm of it.
The trading rule of avoiding doing anything flashy until you get into the flow of things is not limited to trading; it is an idea you can see in all businesses. Trade conservatively until a rhythm develops that you can recognize and exploit, and THEN join in. Do not be greedy and get yourself stuck early.
But how do you avoid digging yourself into a hole?
Have a risk management strategy.