Obeying God's will is actually more simple than it seems. The truth is that some actions are good (holy), some actions are bad (sinful), and the majority of actions are completely neutral. Driving your car is an action that is almost always completely neutral. Walking to the store is an action that is also almost always completely neutral. In fact, almost all actions we do are neutral actions, being neither good nor bad.
This is why God gives us great flexibility in making many decisions.
Note: If you read the article on knowing when God is speaking to you, then you will have read that sometimes no news from God is good news.
Furthermore, when actions are good to do, it is often not sinful to not do them. Brushing your teeth may be a good thing to do, but it is not a sin to never brush your teeth (some people do not even have the means to brush their teeth). Exercising daily may be something good to do, but it is not a sin to never exercise (what if you are paralyzed?).
Some people may argue that if you can do something good, it is wrong to not do so, and in fact there is a verse that mentions this concept (James 4:17), but in context James is discussing issues that matter like feeding the hungry, clothing the naked, and caring for the sick.
James is not talking about how you put on your clothes, or whether or not you brush your teeth. In fact, James would probably accuse such a person of being narcissistic, because they are concerned about petty, selfish actions instead of selfless actions like feeding the hungry, clothing the naked, and caring for the sick.
Paul discusses this fact of the neutrality of many actions when he talks about eating meat sacrificed to idols. He says, "But food will not commend us to God. For neither, if we don't eat, are we the worse; nor, if we eat, are we the better." (1 Corinthians 8:8 WEB) What we eat is a neutral action, even if there are better foods for us to eat and foods that are bad for our health.
Anyone who wants to argue that eating any particular food is sinful has missed the kingdom of God, much like the pharisees. Jesus told them, "Woe to you, scribes and Pharisees, hypocrites! For you tithe mint, dill, and cumin, and have left undone the weightier matters of the law - justice, mercy, and faith. But you ought to have done these, and not to have left the other undone." (Matthew 23:23 WEB)
God allows us great flexibility in making many decisions on our own (we do have freewill), but there are some decisions where God is extremely strict about what is right and wrong. These actions are made clear in the bible, both in the ten commandments and by the teachings of Jesus Christ.
Paul summarizes a list of bad and sinful things for us by saying, "Or don't you know that the unrighteous will not inherit the kingdom of God? Don't be deceived. Neither the sexually immoral, nor idolaters, nor adulterers, nor male prostitutes, nor homosexuals, nor thieves, nor covetous, nor drunkards, nor slanderers, nor extortioners, will inherit the kingdom of God." (1 Corinthians 6:9-10 WEB)
So the bible is clear about what not to do, and the bible is also clear about what good and holy things we should do: "What good is it, my brothers, if a man says he has faith, but has no works? Can that faith save him? And if a brother or sister is naked and in lack of daily food, and one of you tells them, 'Go in peace, be warmed and filled;' and yet you didn't give them the things the body needs, what good is it? Even so faith, if it has no works, is dead in itself." (James 2:14-17 WEB)
The words of Jesus also confirm this truth that we show our faith by what we do when he tells the Parable of the Sheep and Goats, saying, "Then the King will tell them on his right hand, 'Come, blessed of my Father, inherit the kingdom prepared for you from the foundation of the world; for I was hungry, and you gave me food to eat; I was thirsty, and you gave me drink; I was a stranger, and you took me in; naked, and you clothed me; I was sick, and you visited me; I was in prison, and you came to me.' Then the righteous will answer him, saying, 'Lord, when did we see you hungry, and feed you; or thirsty, and give you a drink? When did we see you as a stranger, and take you in; or naked, and clothe you? When did we see you sick, or in prison, and come to you?' The King will answer them, 'Most assuredly I tell you, inasmuch as you did it to one of the least of these my brothers, you did it to me.'" (Matthew 25:34-40 WEB)
These good things to do are what James is talking about in James 4:17 when he says, "If anyone, then, knows the good they ought to do and doesn’t do it, it is sin for them." (WEB) And yes, it is sinful to neglect to do these good things. The words of Jesus also confirm this truth, since refusing to do these good things is the reason given to the goats (meaning, the unsaved) for why they are cursed to the eternal fire. "Depart from me, you cursed, into the eternal fire which is prepared for the devil and his angels; for I was hungry, and you didn't give me food to eat; I was thirsty, and you gave me no drink..." (Matthew 25:41-46 WEB)
So obeying God's will is actually very simple. Do what is good (love your neighbor), avoid what is bad (do no sin - just obey the ten commandments), and don't worry about the many neutral decisions. God will tell you if he cares about your choice on a neutral decision, as discussed in the article How to Know When God is Speaking to You.
If you are truly uncertain about a decision or action, then ask yourself this question, "Is this action something God would want me to do?"
If the answer is even just 'maybe' yes, then it probably is an action God would want you to do. If the answer is even just 'maybe' no, then it probably is something you should not do. Typically, we know whether or not an action is right or wrong, but our selfish desire to do the opposite is why we are unsure. For example, we may know that flirting with that married Christian is wrong, but we doubt if it is because we selfishly want to flirt. If you can at least say that 'maybe' this is something God doesn't want me to do to, then it is almost always 'absolutely' something God does not want you to do.
Even if you are truly unsure about an action, it is better to be overly cautious than to accidentally sin.
"He has shown you, O man, what is good. What does Yahweh require of you, but to act justly, To love mercy, and to walk humbly with your God?" (Micah 6:8 WEB)