Click the start button, select "All Programs," navigate to the "Image-Line" folder and open FL Studio. Displayed on the page will be the step sequencer, which allows you to create sequences for individual patterns; the playlist, which lets you arrange the patterns to build up the structure of the song; and the browser, which enables you to browse through different instrument samples and sounds.
Click "Channel," located on the top toolbar, navigate to "Add One" and select the plug-in you'd like to use. The list includes a range of default plug-ins, each suited to different purposes. FL Key is centered around piano sounds; FL Slayer, guitar sounds; Wasp, harsher electronic sounds, and so on.
Adjust the settings to change the way the channel sounds. Play a note on your external keyboard/MIDI controller to preview the sound. Alternatively, click the Piano Roll icon, also located on the top toolbar; it is labeled with a set of five piano keys. This icon will open the the Piano Roll interface, where you can construct melodies; here, you can use your mouse to click a key along the side and preview the sound.