Hi - I didn't understand this myself, sadly missed out of decades of potential gains..
Ok - so what ive come to learn is that the best ISA's is you don't mind a bit of risk, and plan to invest money for about 5 years, is a stocks and shares ISA... I personal use Interactive investor, but there is Hargreaves Lansdown and Vanguard. They all charge for giving you access, and the cheapest is Vanguard at 0.15%pa of your funds invested. Interactive Investor is £9.99 a month if I recall correctly...
So - my first advice is given that you are hinting at your age and thus probably a little later in life, but willing to accept some risk, then Vanguard will be a good starting point for you...
Once you have selected a provider, then you have to (a) put in upto max of £20k per annum, but I think you can put in as low as £100 a month... then (b) define an investment strategy... But you can take your time with (b) as once the money is there, you can take a bit more time...
To get you started, have a look at index linked funds, these add a bit more charge another 0.2% usually, but generally, these charges are lowest on the market. (Its Vanguards biggest selling point.). Have a look at the index funds (I'd suggest US or Global) and basically use the money you've put in the ISA account, to buy that fund, and I think there's options to do that automatically...
Other platforms will charge you transaction fees for doing that, but Vanguard funds in a vanguard ISA, you don't pay transaction fees.
You should find then, that you make some return on your investments...
Alternatively, if you have £40k sitting around, and you don't need access to it, get £20k into the same Vanguard ISA asap, and in April you can put in the other £20k... I would suggest a different selection of funds to choose in the case of large deposits, perhaps pick 2 or 4 funds that don't have much overlap, and go with that.
To reiterate the main point though, if you do invest in an ISA, no matter which kind, really you are locking your money away for 5 years+, and although you can access it, you will risk selling funds at a loss.. In reality, that is the same disclaimer you see everywhere, and people do make real losses, but in reality you will probably make small profit quickly and after 2 to 3 years see the benefit... If you do get hit with a pandemic, a recession, a market crash, you will see profits eroded and probably be at a loss... HOLD YOUR NERVE.... and don't sell..
The key to making money in the markets, is diversification, a bit of luck, not having all you eggs in one basket (although small amounts < £5000 and in funds I'd argue that doesn't apply) and not selling....
I would also begin the lengthy process of learning about investing... Vanguard doesn't give you access to everything out there, but later on you can transfer your holdings to one that does... Then you will be able to invest in precious metals, individual companies, bonds, play currencies off against each other... But that's not relevant to now for you...
To sum it up, the most simple thing I can say, is get a Vanguard ISA, start putting in £100 a month into a US or Global, Index Based Equity Fund...
The most I've ever lost was 15% of my investment in a crazy Russian fund I took a massive gamble on in the 90's... And whilst the headlines say you can lose money, its real hard to do that if you stick within the suggestion I've made above for you...
Good luck