What is a flat big silver bug with lots of legs and how do i kill it?

You're most likely looking at a silverfish or a firebrat.

Silverfish and firebrats eat a wide variety of foods, including glue, wallpaper paste, bookbindings, paper, photographs, starch in clothing, cotton, linen, rayon fabrics, wheat flour, cereals, dried meats, leather and even dead insects. Silverfish often live in damp, cool places such as basements and laundry rooms. Sometimes, they are found in a bathtub, sink or washbasin, unable to climb out. Firebrats live in hot, humid places such as attics in summer and near furnaces, fireplaces and heat pipes in winter.

You can kill them with strategically placed pheromone traps, putting pesticide into wall voids via holes behind your light switches. Putting insecticides or pesticides near cracks, at the bottom of bathroom walls, and the like works good as well.