Buy (or download) and install a recording program compatible with your computer. Begin working with free programs like Audacity before you spend money on a more in-depth program such as Logic or Acid Pro.
Plug your MIDI/Audio interface into one of your computer's USB ports. Plug your USB microphone into another one of your computer's USB ports. Follow the pop-up directions to install (if there are any. Many microphones are automatically recognized by computers). Plug your MIDI keyboard into another one of your USB ports if you have one left. Swap the keyboard out for either the interface or the microphone if you're short on USB ports. You'll learn in the following steps which device to use when.
Open the multi-track recording program that you installed in Step 1.
Create a new track by clicking "File" and then selecting "New Track" from the drop-down menu. Find the input for this specific track and then select the MIDI keyboard. Choose one of the drum pads for the keyboard sound and click "Record." Experiment with the keys to find which keys play which drum sounds.
Plug a bass into your USB/Audio interface with a standard one quarter inch guitar cable. Create a new track, this time selecting the audio interface as your input. Lay down the bass guitar over your drums by hitting "Record" only on the bass track. You should be able to hear the drums while you play bass.
Plug your guitar into the USB/Audio interface if you want to use real electric or electric/acoustic guitar. Skip this step if you're not laying down guitar. Make sure the input for this new track is the Audio interface and record over the bass and drums.
Create another new track but this time select the microphone as your audio input. Wear headphones to prevent the song from being picked up in the background of the vocal track. Lay down the vocals over the rest of the tracks just as you've been doing. You can make as many vocal tracks as you like, repeating this same step.
Add synthesizers, piano or any other bells and whistles at the end of the song through the MIDI keyboard. Just repeat Step 4, only substituting whatever sound you want for the drum pad.