ETF index funds that are heavily diversified across both sectors and geographic regions are a great place to start - just make sure the total expense ratio stays below something like 0.3%.

Personally, I’ve set mine to automatically invest €500 each month, regardless of what the market is doing. I never try to time it - I just keep buying and never sell. The ones I'm personally buying are iShares EUNL and IS3N with 80/20 distribution.