You have two main ways to start this kind of argumentative essay. You could pick which argument you personally believe more, or you could pick which argument is easier to defend.
If you feel passionately about the fact that Buck should or should not answer the "Call of the Wild," then do that. But if you don't care either way, definitely pick which one you think will be easier to defend. The fact that you think it is easier to defend means that you already have in your mind bits of evidence that you would use to support your argument.
Once you have decided which side of the argument that you stand on, you need to write a thesis statement to guide your reader through your argument. Since the writing prompt is an "either/or" type question, use a compare and contrast thesis. Start with the word "although."
"Although Buck's life might be easier living in the world of humans, it is more important that he follows the call of the wild."
You could flip that around, if you feel the other way. Regardless, you now have the set up for a two part essay. Your first job is to tell the reader why Buck's life will be easier in the human world. Use the the beginning of chapter one to show how good Buck's life was with a good owner. The second part of your essay needs to prove to your reader that while the first argument is valid, it is still incorrect.