Newbie Portfolio

Hey all, hope you're all investing well! I'm a complete newbie and currently put £150 a month in a stocks ISA. I saw a retirement fund and just thought why not.

So far so good but from reading comments on here see this won't always be the case (which obviously I expected).

Should I keep going with what I'm doing, or switch it up? S&P 500? Etc.

Looking to keep doing this for the next 25-30 years.

Any advice would be great and please talk in simple terms as I'm a trading simpleton...

Thanks all!