Familiarize yourself with the layout of FL Studio. The window on the left hand side is called the Browser. Use this window to select instruments/effects. The window in the middle is called the Channel Window. The layout is like a step-sequencer, meaning that it is capable of looping a pattern of specified notes. This window is where most of your beats are made.
To start making a beat, click the empty rectangle to the left of each lane in the Channel Window. This will allow you to select a sound from the drop-down menu. For each empty row, you can select an additional sound. Make sure the sounds you select will mesh well together.
Press the "play" button on top, middle portion of the menu bar. Alternately, press spacebar to begin playback. You should see lights lighting up in the Channel Window, but hear no sound.
Click on an empty square in the Channel Window. You should begin to hear one of the sounds you have loaded earlier being played back. Click multiple squares in different rows and columns to hear how the different sounds you have selected will mesh together. You can change the sounds in real time as FL Studio is playing back your beat.
Open up the mixer by clicking the mixer button in the top, right corner. In the mixer control window, click and drag the sliders to control the volume of each respective track. Each channel should have a name corresponding to what sound it is modifying. Make sure your mix is balanced and is pleasing to the ear.