I think you need one of these

Its know 2.5 to 3.5 jack converter and can be found in any store that sells music or video systems..
1 more short note its not a good idea to plug mic in your amp. and use it as a medium to route to PC as Im pretty much sure you will get disturbance in you mic.
And i think there are chances by a long term use the sound card will be spoiled..