English puntuation help

So. I am doing AQA in Year 10 as I got a D+ in an AQA past exam. (Is this a bad thing?)

However, I am still abit unsure on where to place commas and semi colons in sentences. I have looked online and find them very confusing!

I know to put them in lists (E.g I want egg, sugar, butter, flour and juice) and sentences like this

"However, I would like to know more"

Would you use a semi colon like this "Operation failed; return to base".
(I don't know, my friend sent it to me)

Could someone help me?