The G major chord is constructed from the G major scale. When the major chord formula is applied to the G major scale, it can be seen that the G chord contains the notes G, B and D.
Play the three notes of the G chord with the first, third, and fifth fingers of your right hand, individually, and then together as shown in the following example.
Once you are confident you know the G chord, practice changing between G and C as shown here.
Another way of creating an interesting keyboard part is to accent (play louder) some of the notes as shown in the following example. The accents do not have to be written in the music, it is often left up to the individual player to decide where to use accents. An accent is indicated by a wedge mark placed above or below the note to be accented. In this example the accents are played along with the bass and bass drum.
If you are playing with other musicians, it is important to listen to what they are playing and to play a part which fits with what they are doing. In the next two examples the left hand part coincides with the bass and bass drum and the right hand part coincides with the snare drum. Experiment with other parts which work along with other instruments.