Well, a lot to navigate. I invest via the AJ Bell Youinvest Platform but I did read a few thick books to get my head around the basics.
If it all feels a bit much, have a look at Vanguard
www.vanguardinvestor.co.uk/investing-explained/what-are-lifestrategy-funds
I've got some money invested in their funds and have been pleased with the performance.
You can choose different levels of risk. The more risk you take, the more money you can make. But you need to balance that out with the amount of risk you're comfortable with.
You don't just 'lose all your money' - it's investing and not gambling.
Even if you go for the high risk option, your money is spread across hundreds/thousands of companies from around the world. Some might go bust but others will do very well so it balances out in your favour.
The realistic risk is that it all goes in cycles, so if there's a recession, you might lose 20% of your high risk investment. However, when things pick up, you might as well gain 50% in 3 years. Overall, you'll be set to make a reasonable profit (many times over what you'd get in a savings account).
You just need to make sure that there is some flexibility to when you pay the money out so you're in a position to wait for a year or two if the markets happen to be down. Or, if you know you'll need the money in June 2025 and things are doing very well in 2022, you can then move your money to a lower risk investment (won't make that much money, but won't lose much either).