Purchase a digital audio workstation such as ProTools or Reason that will allow you to create rap beats using the included synthesizers and drum machines. Digital audio workstations offer a variety of mastering tools such as equalizers and compressors that will help create professional results. Links to the previously mentioned products' websites are included in the reference section.
Create your beats by recording multiple layers of drums and other sounds on a synthesizer keyboard such as the Roland Fantom or the Korg Trition. These keyboards include a catalogue of sounds that are commonly used in hip-hop and rap music. After recording your beats, they can be transferred to a computer using the audio and midi connections built into the keyboards.
Listen analytically to popular beats or backing tracks and decide what elements of the music you find appealing and would like to emulate. Use your hardware or software to make a "copy" of the beats you enjoy, exploring your equipment to find sounds and features you may have not previously encountered. This method of music creation will expand your artistic palette and will help add variety to your beats.
Share your beats with other musicians to gain valuable feedback on your products. Constructive criticism from other musical artists will often help you refine your music and create engaging, professional results.