Create a list of the software that you want to install on the computer. Write on the list all the things you'll need, such as DAW, effects or software instruments. Don't forget outboard gear that you want to connect to the computer, like analog mixers or synthesizers. Note what the recommended system requirements are for each piece of equipment. You don't want the minimum because that's a recipe for running into all sorts of problems later on.
Open the computer tower that will fit all the components you need. Plenty of power and USB ports are desirable. A couple Firewire ports are also advantageous because they transmit data faster than USB.
Attach the motherboard to the tower. The motherboard, or main circuit board, should line up with a series of holes in the tower. Take the Phillips screwdriver and screw the motherboard securely. You'll be limited here because certain motherboards will only accept certain CPU and RAM capacities.
Install the CPU and fan using the screwdriver. As far as CPU type, the main choices you'll have here is Intel or AMD.
Insert the RAM, ensuring that the gold contacts are securely seated and the clips firmly in place. Choose the latest, fastest RAM you can afford. You can't go overboard here. The more, the better. Attempt to acquire double your needs if possible.
Install the hard drive. With sound quality increasing, file size also increases. So the more storage capacity, the better. If your budget will allow, more than one would be preferable, using one as a backup.
Install the sound card. Seat it firmly in the motherboard slot. This will be the most important part of your studio setup. Do not get a consumer sound card. Professional cards have better converters and other added features that consumer cards won't have. Close the computer case and turn on the computer. Install the sound card driver.
Connect the studio speaker monitors. These should be professional studio monitors, not computer speakers. Take your time choosing as you'll more than likely be stuck with these for a long time. Where you live (apartment or house), what kind of room you're recording in and what you can afford will determine what specific speakers you purchase. Ask the music sales representative and try to get a live listen to the monitors in action.
Install the DAW. This will be the center of your music creation system. Depending on how serious you are and what you can afford, you should lean toward one of the Fab Five: Sonar, Cubase, Digital Performer, Pro Tools or Logic. Others to consider that you can use in conjunction with these as well as stand-alone are Abelton Live, FL Studio and Reason.