All computers of recent vintage are candidates for music recording, but more powerful units are recommended to handle the large processing and storage requirements of music data. Choosing a recording computer or determining if your machine is suitable will be determined by the music software's system requirements, which are listed on the computer's box or the manufacturer's website.
Music recording software is available in many different configurations, with varying feature-sets and complexity of use. Some are basic recording programs and offer no music-making capability of their own, while others offer recording, built-in musical instrument sounds and sample libraries. Prices vary considerably -- from free open-source recording software to professional programs costing thousands of dollars.
Computer sound cards are limited in their capacity to produce high-quality recordings or handle the varying input signals of recording microphones and music instruments without distortion. Recording interfaces either modify the input signal to enable higher-quality results from your existing sound card or serve as studio quality sound card replacements. More expensive interfaces offer built-in preamps to further optimize sound include standard musical instrument and microphone connectors and are connected to the computer via FireWire or USB. Many interfaces are bundled with proprietary software, eliminating the need to purchase it separately.
Studio microphones are quite different than computer microphones and can cost from less than $50 to well into the thousands of dollars. For most home users, a collection of two or three moderately priced studio mics for vocals and instruments will yield good quality results. USB studio microphones are a value-packed beginner's alternative and combine a studio microphone with built-in USB recording interface, eliminating a sound card upgrade or dedicated interface.
Higher-end recording interfaces include MIDI inputs for connection to MIDI compatible devices such as keyboards or other digital controllers. MIDI (which stands for the technology's formal name, which is Musical Instrument Digital Interface) is widely used with recording and music production software to enable control over the software's built-in sounds and instruments. If you are recording traditional instruments and voice with microphones, MIDI is not necessary and may be added later if needed.
Your computer speakers are not capable of producing all the sonic nuances that quality recording requires. Invest in the best set of powered studio monitors (those that require no external amplifier) and headphones you can afford.