Before you begin the first exercise, count 1 2 3 4 1 2 3 4 several times to get a feel for the rhythm. As you play, count mentally as you play and tap your foot to help you keep time. Count 1 2 for the first half note in each bar and 3 4 for the second half note.
To be sure you develop a good sense of time right from the beginning, it is recommended that you always practice with a metronome or drum machine. Count along with these beats to help you establish the right tempo (speed) for each example.