Go to the Image Line website (See Resources) and download the latest version of FL Studio.
Install FL Studio and load the program. Click the icon that has four buttons on it under "Click to Enable Online News". This will bring up a beat pattern composer. Click on the squares next to "Kick", "Clap", "Hi Hat" and "Snare" to compose a beat sequence.
Click the vertical icon on the main menu and drag sounds from the the catalog over to the beat maker. Click the play button icon and highlight "Pat" to listen to your beat for that pattern. Click the square with the number 1 that says "Pat" to make a new beat pattern.
Click the icon to the far left of "Click to Enable Online News" to bring up the Composer menu. Click and drag the squares beside the number of beat patterns to compose your song. Click the play button and highlight "Song" to listen to your entire song.
Highlight a beat pattern and then click on the piano key icon under "Click to Enable Online News" to bring up the Piano Roll. Select a pitch or key and drag the squares for how long you want them to play for that measure.