One of the most important things when making arepas is texture. Texture is extremely important to pay attention to throughout the course of your arepa making. The dough dries out fast, so keep that in mind. The arepa dough should be formed and cooked relatively quickly after forming the ball to insure a moist inside.
When mixing your ingredients, keep some extra warm milk or water on the side. As you form your first couple arepas, your dough ball will start to dry out. Add a little of that extra liquid a splash at a time and fold it in. This way your arepas will keep a solid consistency from the first one made to the last.
There has been a long debate whether to use water or milk. Some people stand on each side. Milk can add creaminess and allow the dough to stay moist longer. Water is simple and for the most part readily available.
Creating the perfect arepa dough will be a skill you learn over time. Have fun and try to experiment with amounts.