How can I summon Baron Pit

You can't summon Baron Pit directly — it's a battlefield token, not a card you can play or include in your deck. The only way it enters the board is by playing Baron Nashor (Chaos, 10 energy).

His ability is a replacement effect: "As you play me, add the Baron Pit battlefield token to the board if it's not there already. If you do, I enter there." So when you play Baron, the Pit appears (if it isn't already on the board) and Baron enters it instead of the battlefield you played him to. You'll then typically conquer the Pit since it's a new battlefield you've gained control of (Rule 369.3, FAQ #9221).

Two notes: only one Baron Pit can be in play at a time (the ability checks "if it's not there already"), and the Pit's token text itself says "You can't start the game with a token battlefield." If the Pit is ever removed — e.g., replaced by a Brush token — playing Baron again will create a fresh Baron Pit (FAQ #10152).