One piece of advice that I learnt over the past years is to not fall for the home country bias.
Being German and believing in a growing domestic economy, I bought DAX ETFs (representing 30 major German companies). Seeing the volatility of the DAX over past years, I could clearly see the difference between an ETF representing 30 companies (DAX) and one representing 1600 companies (MSCI World). While the MSCI World grows slowly and quietly, the DAX is going up and down. Being interested in slow and steady long-term growth, I clearly prefer to stay away from volatility-prone investments in the future.