Expanding the rib cage is a good way to make the chest look bigger. The intercostal muscles (those between the ribs) and cartilage are quite flexible when we are young, and they can be stretched by doing pullovers.
Holding the dumbell at the streching point is most important, with a deep breath.
As for broadening the shoulders, well, that's all genetic. However, you can do lateral raises (full range of motion) to give a wider appearance. Start with the dumbells at your side and raise them up at least to your head level, slightly turning the wrists, as if pouring out water fom two jugs. You don't need a lot of weight for either of these.
Good luck!