What is the best way to resign from my current job?

Well, for starters, I hope you kept a copy of the contract. You need to comb through it and find the penalties for early termination. Most contracts have a clause allowing you to terminate with written notice without requiring you to give any reason. If yours does, then when asked, simply state "I am exercising my right." It is important that you firmly stick to that statement. It will protect you professionally. Even years down the road when in another, similar situation, when a potential employer sees that you have 'exercised your right' they know that any non-disclosure agreement will be respected. You will find your way out of the contract in the doent itself. The employer always leaves themselves a way out. Your way out will piggy-back on it.