It may help if you try to find a job such as dishwashing to start off with. Yes it's horrible, slave labour but my application literally consisted of the following phonecall:
Me: "Hi, I was wondering if you needed any dishwashers at the minute"
Them: "Yeah, we'll put you on the rota next week"
Me: "OK thanks"
If you stick at it for a few months it gives you something to put on your CV, which will give you a higher chance of getting a job when you turn 16.
I am 15 soon to be 16 in December i really wanted a summer job. And i also havent got my NI what should i do