How to file US Taxes and Back taxes - Korea

To put it simply, every US citizen must file taxes annually.

The Foreign Earned Income Exclusion limit for 2017 was $102,000.
Although you may not have earned above this amount, you MUST file taxes so that the IRS can verify your income is less than this amount.   In some cases, you can request bank statement to verify your income.    
For your current tax year, many people use Turbotax - Freedom edition.  This will give you the option to file electronically.  And it's FREE.  

In regards to back taxes, I would recommend contacting a tax advisor here in Korea that has experience with these types of situations.  They will file a "streamline" tax return and you will send a letter stating your reasons for not filing such as: "I thought I didn't need to file because my income didn't exceed $102k."   

Based on the estimates I have received, it ranges from about $100-$200 per year.  H&R block seems to be on the higher side of the scale.  Although it's quite expensive, I would recommend doing it asap.  The IRS isn't a bill you want to delay.  

Korean taxes are generally completed by your employer.  You will need to get on internet explorer and download your banking certificate on a USB so that the process can go smoothly.  

Or you can go to a local tax office with your income statements, id, housing contracts, verification of rent deductions, etc.. 

State taxes are generally not filed by expats. 

I hope this information helps put you in the right direction.