Really it's just figuring out what you enjoy doing and connect it to a field and trying things out.
A lot of people have trouble deciding.
I know you want to do A levels, but you could always try going to some taster days for BTEC courses, since they're practical you could see if it would be something that you would possibly enjoy doing. Then select the A levels accordingly.
Just try as many things as you can and see what you enjoy.
Over the Easter break