Typically, silicone mouth pieces will be most flexible and comfortable, but will cost more. You get what you pay for. Finally, don't use a fancy device to snap your snorkel onto your mask. This basic rubber band-like device will keep your snorkel in place and reduces the chance of you losing it. Your answer will determine what kinds of fins you purchase.
Fins have a broad price range and vary greatly in their materials, shapes, and sizes.
In general, the longer the fin and the stiffer the blade, the more the fin empowers your kick. Furthermore, vented-blade fins are designed to increase power based on the flow of the water through the vents. Comfort and fit should ultimately help you determine your purchase. We'd prefer not to get into what all this stuff does. You'll learn it in your certification course.