Wanting to join at 17

Currently in my 1st year of college and when I hit my second year I want to drop out ( I have to do 3 years) and join the RM it’s always been my dream. I have my GCSEs but education has never hit me I have my maths and English but I want the RM! So I’m going to get super fit this year and join at 17. Can you give me advice please on training I want to join so badly I feel so depressed in school