Go to Options menu and select Project Info. Type in a title for the project.
Open the Mixer panel.
Select one of the unused tracks. Rename the track to something description by right-clicking on it and choosing Rename/Color
To the right, where you see an <IN> tag, select one of the input sources. You will only see options in this list when you have any mics or instruments connected to your computer.
When you select an input source for a track, you'll see an "Arm Record" icon (little floppy disk icon) lights up below the track in the mixer. Because you followed the two naming steps described above, the tracks will be recorded into a folder and named automatically. Recorded tracks can later be accessed under the Recorded folder in the browser window of FL Studio.
When you're ready to record, click on Record button in the transporter window. A dialog box will pop-up asking you how you'd like to record. Audio, into the playlist as an audio clip, is the option you want in this case.
By default, the tracks will be routed to the Master track. But if you want to route them to other tracks, for mixing, you can route a track to another one by selecting a track in the mixer window, than engaging the little Up arrow button on another track.
Sources:
http://warbeats.com/Tutorials/FL-Studio?topic=13268&name=FL-Studio-Basics-12-Recording#view