When you open up FL Studio, you will be presented with the Step Sequencer. You are ready to start creating drum patterns right away.
By default, FL Studio adds four drum sounds at launch, the Kick, Snare, Hi Hat, and Clap.
The Step Sequencer contains the name of the track on the right.
And on the left, it contains 16 boxes. These boxes are alternately color coded to divide the pattern into measures. Each measure has four beats, by default.
To have the drum sound triggered for a specific beat of a specific measure, toggle the box corresponding to that beat in the Step Sequencer.
Experiment with toggling different boxes for each of the four initial drum sounds.
Click the space bar on your computer keyboard, to play your newly created drum pattern. Click the space bar again, to stop the playback.
Ready to add other drum sounds to the pattern?
Go to the main menu, and select Channels->Add One. This will give you a list of all sound generation plugins available.
One of the plugins that allows to add drum samples, is the Sampler plugin. Select the Sampler plugin from that menu. That will create a blank track ready to receive a sample.
3). Now look at the Browser column
3). Click on Packs in the Browser column. This should give a number of drum sounds. Click on a drum sound that looks good, and drag it to the Step Sequencer on top of the name of the track (which says Sampler at this point). You will know if it's a sample, if it allows you to drag and drop itself into the Step Sequencer. Other things in the Browser column will not be draggable.
The name of the track will be replaced with the name of the sample you've just dropped onto it.
Want to add more sophisticated pre-made drum patterns, like the kind real drummers play? There is a plugin in FL Studio called FPC. It comes with hundreds of pre-made drum patterns you can drop into your project.
To add the FPC plugin, go to the main menu, and select Channels->Add One.
From the list, choose FPC. This will add a new track to the Step Sequencer called FPC.
Select that track.
An FPC plugin properties box will appear. 
Find the Pattern Manager drop down. Select a pattern that looks good.
The corresponding track in the Step Sequencer will be populated with the drum pattern you have just selected.