how to return to us after voluntary departure,

As your spouse is a US citizen, and it appearing that you have a bona fide marriage since you already have a son (I am assuming that your son is born of the union of you and your wife), your wife could apply for you to return as a permanent resident by filing form I-130, having it approved, and then by you consular processing your case at your home US consulate or embassy. If you overstayed your period of time in the US by 180 days or one year prior to the order of voluntary departure, you would likely require a waiver of the three or 10 year bar respectively. Such waiver would be based upon a demonstration that non-granting of such would cause extreme hardship to your US citizen wife. Due to the limitations of the Lawyers.com Forums, Alan Lee, Esq.'s (the "Firm") participation in responding to questions posted herein does not constitute legal advice, nor legal representation of the person or entity posting a question. No Attorney/Client relationship is or shall be construed to be created hereby. The information provided herein by the Firm is general, and requires that the poster obtain specific legal advice from an attorney. The poster shall not rely upon the information provided herein as legal advice nor as the basis for making any decisions of legal consequence.  

That depends on why you were removed and whether you have accrued any unlawful presence.  Voluntary departure doesn't eliminate the bar for an overstay, for example.  You would need a waiver for that.