I really want to fight Plantera, because I have all the gear and stuff. But there's one problem; I don't know how to summon it. What do I have to do to summon Plantera?
You need to find a item in the Jungle called Plantera's Bulb, and destroy it:
Plantera's Bulb is an object found in the Underground Jungle which can be destroyed with a Pickaxe. When destroyed, it summons Plantera.
In regards to how to find it, have a look at this answer: Having trouble finding Plantera's Bulb