2004 Impala SS

I have a 04 Impala SS, I want to purchase underdrive pulleys. Now I have been hearing to purchase smaller pulleys and to purchase bigger pulleys. Does anyone know which one to buy to I can increase the performance. I also have been looking for a tuner, I want to tune my car so I get optimal performance but I dont want to mess up my engine. Does anyone have any ideas or solutions?