1

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?

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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.
Terraria Wiki

In regards to how to find it, have a look at this answer: Having trouble finding Plantera's Bulb

2

You have to defeat all of the Hard Mode Bosses first before Plantera can be summoned. After that, there's a message that says,

The jungle grows restless.

This means Plantera is abled to be summoned. Find a jungle and look for Plantera's Bulb. Destroy the Bulb, and it will be summoned.