Grow lights can be placed above the plants.
Grow lights can be placed at any height from ground level to 15 feet or more. I would recommend that you do not place your grow light directly on the soil as this will cause your plant to burn and eventually die. The best way to position a grow light is to have it over a pot so that the roots are just barely touching the sides of the pot and the rest of the plant is in shade.
This will keep the plant healthy and happy. You can also hang a hanging plant holder with a light bulb in it over a pot.
Any wattage will do as long as it produces enough light to promote growth. A good wattage light would depend on what type of plants you are growing, how much space you have and your budget. There is no "best" wattage light. It depends on your own personal preference and budget. Grow lights are available in a wide variety of wattages, from 1 watt to 50 watts. The higher the wattage, the more light is emitted.