Advising in investments is tricky, advising in sustainable investments is the same.
But here is some information to give an idea.
The main rule or investing your money is, to invest little bits in many places. Never bet on one horse. So, you can put you money in solar panels, windmills, in hydrogen, in smart grid, whatever, but never put all in one thing or company.
The market of financial products for sustainable investmenst offers already a diversity of options.
You can put your money in shares of companies. You buy them in the stockmarket, at a broker. That is rather cheap. You buy could buy stocks from a hydrogen company like Air Products, Liquide, Plug Power. But, you have to dive deep down into the activities of these companies. For example, most of the hydrogen is made out of gas (methane), and that is not sustainable; they should make it out of water, with the use of renewable energy.
You can also invest in funds. A fund charges a percentage of management fee, purchase and selling fees and that is a disadvantage. Advantage is that you can choose funds with each its own mix of investments. Like only solar, or ‘renewables’ which is generally solar and wind. Or funds for water. Often it is the best to go for the conventional sustainable funds. In Holland the ASN bank and Triodos bank have funds.
There are much more funds and investmen varieties. Such as ‘Impact Investing’, which is a broader thing. Such as SRI (Social Responsable Investment) or ESG (Environmental and Social Government Investing). Check this for more about that.
You can also choose for emission rights. In Europe there is an ETS Emission Trading Scheme, set up by the European Union government. In that scheme, a company gets a certain amount of emission rights. If the company wants more to emit, it can buy extra emission rights, from companies who emit less. So the lesser you emit, the more profit you could make. You, as a private, can also buy these emission rights. You get a paper, an ETF, prooving you are the owner. If you are an activist, you simply throw away the paper and there are less emissions to emit. What you see now, is that these ETFs grow in value. Read this for more info.
There are many many sorts of ETFs for sustainability purposes to invest. But realize, they can go up and down.
Most investments are interesting if you put in like 1000 Euro or much more. But nice to mention is that there is the Enerfip , and this already works a bit from 10 Euro. So there is always a start to invest in sustainability.