2 Nephi 2:28-29

And now, my sons, I would that ye should look to the great Mediator, and hearken unto his great commandments; and be faithful unto his words, and choose eternal life, according to the will of his Holy Spirit; And not choose eternal death, according to the will of the flesh and the evil which is therein, which giveth the spirit of the devil power to captivate, to bring you down to hell, that he may reign over you in his own kingdom.

Choose eternal life, according to the will of his Holy Spirit.
When you look at the simplicity of the plan, you can see that it really is as simple as that: choice! We literally have the choice to do good or do evil- it's totally up to us. Then why do we ever choose evil??? The next part gives a little insight into that question.

And not choose eternal death, according to the will of the flesh and the evil which is therein.
The part that is interesting is how this is phrased: "according the will of the flesh and the evil which is therein." So our bodies, being carnal in nature, really do desire evil, by their very nature. The desires of our flesh are inherently evil. But we have the choice. Our spirits can be strong and overcome the desires of our flesh or succumb to them- again, all up to us. Perhaps that is one of the big differences in our resurrected celestial bodies- they will no longer be lustful, lazy, subject to illness, etc.

Which [flesh] giveth the spirit of the devil power to captivate, to bring you down to hell, that he may reign over you in his own kingdom.
Addictions are a powerful captivator of our agency. When we do things that cause us to become addicted, we lose the power to choose. Satan then has us captive and can drag us down to hell, where he will force us to be his subjects in his awful kingdom.