Note that the cosplayers are not in Yoyogi Park, they are on the bridge outside the Meiji Shrine entrance, which you will see soon after you turn right out of Harajuku station.At the entrance to the park you may see the rockabilly guys, then in the park there are people juggling, practicing for plays etc, but I've never seen cosplayers in Yoyogi (and I go there often.) There are cherry blossoms down the main road inside the park on the right hand side, but they are also spread around the park, so just have a look around. Expect any areas with cherry trees to be quite busy, even in the morning- it can be hard to find somewhere to sit. The stalls should be setting up around ten or so, they don't open right at a specific time like shops.
To get to Yoyogi, just follow the crowds basically- go to the right out of the main station entrance (not the Takeshita entrance), cross the bridge and pass the entrance to Meiji shrine, continue right around the corner past the pedestrian overbridge and you will see the park entrance.
In any case, I assume you are taking a guidebook or printing out maps of the areas you want to go to? That should make it easier.
