Direct conversion of the Vinyl discs is not possible. You will need a device which can play the discs that can be connected to the Line In input on the sound card and then using Audacity you will need to record the input signal. Once the signal has been acquired and the track is recorded. Before doing this, make sure to select the playback device and the recording device in the configurations page of the Audacity software.
Press the record button, start the Vinyl disc and wait. You will have the track displayed in signal spectrum. Once the operation is finished, select File > Export as and choose MP3.