I've heard a lot about the effects HRT (M2F) have on your skin and body. Exercising for an hourglass figure can be achieved but it all depends on genes. That goes for hormones as well! I've seen two common body shapes; apple and banana. I am a teenager (15) and a lot of people have said I'm pretty lucky to be starting at this age. My question is, will my pepvis broaden or widen? Will I always have broad shoulders?

Since you're starting HRT in your puberty your pelvis should widen at least a bit. As you said "how much" depends on your genes, but I'm pretty sure It will do. About your shoulders is kinda possible they will narrow a little since you could lose some muscle mass (not sure if you have any, you are way too young). Anyway, thanks to pelvic widening even if you don't lose muscle mass your shoulders will look narrower.

I don't have firsthand experience, but I've seen plenty of photographic evidence of trans ladies who had rather hourglass looking figures after beginning HRT even in their late teens to early twenties. I'd say you  are very likely to develop very feminine features over the next few years. I doubt you'll ever be mistaken for a man.