Make sure the room you're working in is carpeted. This is the first step to soundproofing the room. If you're making a corner studio or working in a closet, purchase a thick rug or carpet remnants to put down instead of carpeting the whole area.
Add foam material to the walls and ceilings by nailing or adhering to the wall with strong glue. Your options include studio-specific foam which is available for purchase online or simple egg crate mattress pads if you're building on a budget. The foam walls dampen the background noise enough for your recordings to stay clear.
Decide on a door option, if applicable, for your studio. If you're using an existing door, purchase some weather stripping and apply to the outside of the door so that you don't get noise leakage. If you're building a studio, purchase an all-weather door, like the ones on the entrances to your home. Make sure that you have some ventilation; you can build a fan box in the top of the studio or if you're using a whole room just run the air conditioning to cool the room before your session.
Purchase or use a computer with a large hard drive. At the least you need a hard drive with 120 gigabytes and you'll need at least 512 MB of RAM. This will guarantee that you have all the disk space needed to mix, edit, and record your music. Having a large hard drive will also allow your computer to keep up with you and your studio work. Make sure you get compatible recording software, and don't go cheap if you want professional sound. You can find inexpensive software options for outfitting your studio, but you'll want to make sure your computer equipment can keep up and perform to your standards.
Buy a bag of zip ties to keep the room's various wires and cords from tangling and making a mess. You may also want to make a few baseboard covers. This is done by simply measuring the length of your walls and building a cover made of wood--think upside down L-shaped. These will keep the wires and cords covered which allows for a clear working space. Once you have everything wired and hooked up place the zip ties around the bunches to keep them organized.