Your (former) employer will issue W-2's by the end of January 2018. Make sure the employer has your current address so that it can be mailed to you.
DO I NEED TO FILE A TAX RETURN?
Who has to file?
you must file a tax return if you have more than $12000 of taxable income (for single; other limits apply for other kinds of taxpayers). Your employer should mail you your W-2, make sure they have your current address.
If you made less than $4,050 maybe someone can claim you as a dependent. Did you live with anyone? How old are you? Are you a student?
What did you sell? It might need to be reported. If it's just your own personal stuff you don't have to report it. Is it something you made or bought to sell?