New to investing

Hi there, I’m currently 18 still at school and have around £11,000 sat in a normal account. I resell clothes and make around 20/25 thousand a year and am looking for somewhere to put the money I make. How can I know the best time to put my money into sp500, what platform should I use and also how much would you put in. Also do any of you have any advice on how to learn trading more? Thanks