In the areas where the sun naturally hits: the upper forehead near the hairline, the cheekbones, the bridge of the nose and the jawline. A classic trick is to draw an imaginary three on each side of the face, from the temple to the cheekbone and from there to the jaw. Always blend the edges and bring a little leftover product down towards the neck so there are no colour lines.
They serve different purposes. Bronzer adds warmth and a healthy glow, mimicking the tone the sun leaves behind, and usually has a warm undertone. Contour, on the other hand, creates shadows to sculpt the face, which is why it uses cooler, greyer tones that mimic a real shadow, and is applied under the cheekbone, along the oval of the face and on the sides of the nose. They can be combined, but they're not interchangeable: using a warm bronzer as contour tends to look orange.
As a general rule, if your skin is combination or oily, bronzing powder will work better for you, as it adds warmth while helping control shine. If your skin is dry or has noticeable texture, choose a cream or liquid bronzer: these blend into the skin, don't cling to dry patches and leave a dewy finish. Normal skin can alternate between formats depending on the finish you're after each time.
Choose a shade one or two tones darker than your skin, never more: the goal is believable warmth, not a colour change. Also consider the undertone: fair skin looks great with neutral or slightly pink-toned bronzers, medium skin works well with soft golds and terracottas, and deep skin needs rich, coppery tones so the result doesn't look grey. If in doubt, start with a small amount and build up the colour in layers.
It depends on the format. Cream or liquid bronzers always go after your base and before powder, since creamy textures don't blend well over powdered surfaces. Bronzing powders, on the other hand, are applied after setting the base with powder, or directly over the base once it has settled if you don't use a setting powder: over a damp base, the pigment can cling in patches and look uneven.