Study artists whose beats you enjoy and try to pick out what they do. Pay attention to the types of sounds they make. For instance, do you like a regular snare drum sound, or do you prefer the clapping sound? Do you like fast beats on the high hat, or do you prefer slow beats? You'll probably experiment with all kinds of different sounds later, but this can work as a great jumping off point.
Download and install a professional beat-making software application such as Fruity Loops or Sonic Producer.
Plug in your studio monitors or professional headphones. You don't need to spend hundreds of dollars on headphones, but you'll need to get a decent pair so you can mix properly. The same rule applies to studio monitors. It's not necessary to spend hundreds of dollars, but you'll probably have to spend at least $150 to get some decent sounding monitors. The better your playback, the better your mix will be, and the more professional your beats will sound.
Open your beat-making software and create a new project. Choose a bass drum sound from the list of sounds to get started. Click and drag it from the sound list and drop it onto a track in the loop window. Highlight various squares along the horizontal line of the first track. This will now be your bass drum track. When you first begin, you should highlight squares at even intervals (for instance, on the one beat and the three beat) until you become more familiar with the program.
Choose a snare drum sound and begin highlighting squares at even intervals along the second horizontal line. This second line will now be your snare drum track. Press the "Play" button after highlighting your squares to hear how it sounds. If it sounds a little off beat, unhighlight a square and rehighlight it elsewhere until you find the right spot.
Add more tracks to your loop by adding high hats, crash symbols, bass guitars, synths and other effects in the same way that you added the bass and snare drums in the previous two steps. When you're finished setting up your loop, you can mix the levels of the various tracks by clicking a track to highlight it, then moving the track's volume bar up or down, in accordance with the volume level you desire.