I suggest buying low-fee domestic index-tracking ETFs like "Vanguard S&P 500 ETF" (VOO), using a low-fee brokerage app like M1 Finance.
... or not. Your situation may call for something else. Any specific suggestions people give you will likely sound unnecessarily specific and perhaps arbitrary, unless you have spent a lot of time learning about the modern investing landscape -- you really need to do that for yourself.
And how to do that? Especially for those of us in Europe?
That's one thing I can't get my head around. The ecosystem seems filled with scammers and financial advise salesmen, and my "scam alert" is running on constant overdrive whenever trying to look for any current and actionable educational material.
I have an account with Vanguard directly and research their funds when investing. Currently my US employer retirement fund partner has very low cost access to Vanguard funds I follow already so I am linked into that.
It really depends on your goals and starting point. Avoiding management fees is one of my priorities as well as direct visibility of my accounts and numbers (all offered via Vanguard directly). There are a lot of great, free information sources. Some are directed at people who are paycheck t paycheck and need to start setting like $100/month aside while digging out of debt. Others focus on middle class style employer retirement plan+personal savings like for a down payment on a house. Others focus on those willing to toss thousands on what becomes a bet on the market and YOLO it (not recommended).
I started by just looking at Vanguard funds and using open/free tools to understand terminology and get a grasp of the history or background like lessons learned.
Read "the long and short of it" by John Kay which deals with a lot of the basics around investing for yourself. It's a truly useful book, and I've given quite a few copies of it to people. It is quite UK-centric, but I think the central lessons could be applied in other jurisdictions.
... or not. Your situation may call for something else. Any specific suggestions people give you will likely sound unnecessarily specific and perhaps arbitrary, unless you have spent a lot of time learning about the modern investing landscape -- you really need to do that for yourself.