I don’t have a resource to point you at, but in terms of choosing a JISA, you’d go with the platform that has the lowest fees. I think platforms that even offer JISAs are few are far between, and the best that I could find was Vanguard. (I hope Freetrade offer a JISA in the future; please vote for the idea!)
As for the portfolio to invest in, I’d say follow the general advice for a stocks&shares ISA for yourself, but consider yourself 18 years old if following the "x - your age" guidance on how much of your portfolio should be in equities.
For myself, I believe these things to be true:
- I cannot pick stocks such that I beat the market
- I cannot predict which markets will do well in the future
- There is no secret guide I can follow or advanced learning I could take that could rectify 1) or 2); magical future prediction abilities actually do not exist
Which means I’m 100% in a world stocks index tracker with a low fee, offered by Vanguard.