I use coinbase, dead easy to use even a knobber like me can work it. Took ages to register the account though, and to verify photo I.d. etc, but I suppose that's good in a way.You use an exchange. You buy whatever crypto you want for whatever it costs in £s that day. Then when you want to sell you cash out for whatever it’s valued at in £s that day. It’s essentially like stock trading.
Some of the less mainstream coins are a bit more complex but if you’re a novice just download Coinbase or Binance (personally find Coinbase easier to navigate) and get purchasing.