What happend at the end catching fire??

Finnick was calling Johanna probably to make sure that the plan was going well, since they were both in on it. Johanna removed the tracker from Katniss' right arm. The "plan" was to do whatever it took to get Katniss out of the arena alive (not Peeta).

On to the whole confusing tree scene, Beetee had a plan to somehow break the forcefield around the arena using the electricity from the lightning, and when Katniss sees the arrow/spear Beetee had, she realizes that she can shoot the arrow into the "weak spot" of the forcefield (the very top). The forcefield breaks and the hovercraft comes in to rescue her.

Because the entire plan to save Katniss was kept a secret from her, Katniss thought that she had to save Peeta. She knew that there could be only ONE victor, so she knew she had to kill everybody else so Peeta could win the Games. So when she sees Finnick, she realizes that she has to kill him too, even though they are allies. When she pulls back the arrow, Finnick reminds her that he is not the true enemy, the Capitol is.

It's not really Peeta they were trying to protect. Katniss THOUGHT that they would try to save Peeta, but Peeta was actually trying to save Katniss in the real plan. Katniss represents hope for a better future for Panem, and they want to use her to overthrow the government and get all the other districts to follow along in their battle.

The game designer, Plutarch Heavensbee, is actually part of the rebellion against the Capitol. He does those bad things to Katniss in order to convince President Snow that he is on his side. He uses that power to help the rest of the rebellion.