A 4/4 time signature means that you will play four beats in a measure and the quarter note gets one beat. You can use a variety of notes (whole notes, eighth notes, sixteenth notes, etc.). Just make sure that whatever rhythm you use will equal the length of four quarter notes.
Use a metronome to keep an even rhythm and count the four beats in a measure to keep your songs on track. You can learn popular songs in Simply Piano to learn more about a 4/4 time signature.