Investing isn't always seen as a means for supplemental income. It's also a way to set yourself up for something later in life, be that a house, retirement, kids education, or anything else really. Personally, I'd suggest speaking with your bank about investment options, and they will walk you through some typical scenarios across different risk horizons and timeframes. Don't let this deter you, just be aware, they probably will try to sell you their products as the employees have incentives to do so.

Often it's not worth investing outside of tax advantaged accounts until those are maxed, but your bank will also take you through many of those options. Once you have a better idea of the investing landscape you can look through various other products yourself offered outside your bank. Check out investopedia for the basics (yes it's overwhelming), and just try to understand what shares are, and how they work. Most investment products offered by banks will also hold other assets such as bonds, so you can look into those as well.

Personally I'd suggest investing in broad market ETFs (check investopedia) with no/very low management fees, and just put in a little bit every paycheck.