Read "Simple Path to Wealth" by J L Collins. Set it and forget it.
Dollar cost averaging into a broad-based US stock market mutual fund or ETF.
I personally invest in stock markets.
mutual fund for long term and I-series savings bonds (max is $10K a year per person)
Ramit Sethi's I Will Teach You to be Rich is an excellent personal finance book that walks you through the different things you should think about as someone investing for retirement.
The writing style is kinda sleazy, but the actual content is dead practical and well thought out.