How do you know which compacitor to use? Since on the compacitor it is measured in micro-farad, how do you know how much amps or voltages it can store?

You mean capacitor. The voltage rating should be part of the specs.

I went to Radioshack and the voltage rating is not on there. I even ask the front desk worker and even he doesn't know

400 Volt and 150 microFarad

Here's a good starting point: http://www.101science.com/Radio.htm#cap

 

More detailed: http://www.allaboutcircuits.com/vol_1/chpt_13/index.html

I went to Radioshack and the voltage rating is not on there. I even ask the front desk worker and even he doesn't know

 

the front desk work probable knows little more than to take the money and handover change

and a receipt, don't expect an engineering mastermind.