What kind of microphone is it? It might require phantom power, which your soundcard won't provide.
Also, you might need to enable the mic input of your soundcard. Some can only use line-in or mic-in, but not have both enabled at the same time. Still others have the same jack handle line-level and mic-level signals, so you have to switch those into "mic" mode, which enables the preamp.
Once you get up and going, you can get reasonable performance from today's soundcards. I still recommend getting something to bypass all this headache. If you get something like the TonePort UX1, you get a dedicated mic input, and guitar amp modeling, all in one. Get the UX2, and you add phantom power and extra connectivity to the mix.
Greg