The type of footbed that you need will depend on why you are looking for an insole. Insoles and Orthotic Arch Supports are available with 4 different footbed types, Rigid, Semi-Rigid, Cushioned, or Flat. Rigid Insoles or Orthotics are extremely firm and will not bend. Semi-Rigid Insoles or Orthotics are moderately firm, but will flex some. Cushioned Insoles will provide you with cushioning but not a rigid arch plate. Flat Cushion Insoles will not have any arch support.
If you purchase a full-length insole, you will most likely need to remove the existing insole from your shoe before placing your new one. Almost all full-length insoles are meant to completely replace the insole you are currently wearing; only very thin and flat full-length insoles can be comfortably worn in addition to your existing insoles. If you purchase a 3/4-length insole, you will place this insole on top of your existing shoe insole. 3/4-length insoles are meant to be worn along with your existing shoe insole. If you purchase an insert piece, you will place this insert either on top or below your existing shoe insole, depending on the specific item purchased (inserts come with instructions for proper placement).