Create a drum beat with a software program such as Reason or Beat Detective. These programs will allow you to construct beats from scratch for any genre including rock, salsa, jazz and blues.
Download samples of bass guitar notes or riffs from mp3 sample websites or create your own by plugging a bass guitar into your sound card. Software such as Emagic Logic or Pro Tools will allow you to record .wav files for use in creating music.
Use digital synthesizer software to create melodies and sound on your computer. You can use modules with these programs to mimic classic keyboard sounds. These synthesizers are able to simulate guitar, horns, bass and percussion sounds, so they are very useful in music creation.
Find vocal parts or speech samples online to add another dimension to the computer music you are creating. A cool vocal line or memorable line from a speech can be used repeatedly in a song to give it character and you can use these samples to create a catchy piece of music.
Construct your song on a multi-track audio production software program. You can create separate tracks and mix them together to create a single stereo track that combines the samples and parts you have created. Programs like Sony's Acid will also allow you to loop samples and extend tracks to make songs of various lengths.