All the examples you have played so far have involved a downward pick motion, indicated by a a. When playing eighth notes, a down (a) and up (s) picking technique is commonly used. This is called alternate picking, and is essential for the development of speed and accuracy. The technique involves using a down pick on the beat (the number count) and an up pick off the beat (the "and" count).
Practice the previous example with the use of alternate picking until you can do it comfortably. Then try the following one, which uses alternate picking for eighth notes and downstrokes for all quarter and half notes. It is a good idea to practice the first two bars and last two bars separately at first, as the picking is quite different in these two sections.
Here are some more examples to help you become comfortable using eighth notes. Remember to use alternate picking and count out loud as you play along with your metronome. Practice each one slowly until you can play it smoothly and evenly. Gradually increase the speed on your metronome until you can play it at a moderate pace.