I made less than $5,000 of taxable income in 2018. However, I invested some of that into the stock market. Do I still need to file taxes for the 2018 tax year?

If you were salaried you could file for any tax withheld and if you are not a dependent you might qualify for earned income credit. If you got a 1099-misc with income in box 7 you have to file to pay self employment tax. 

I didn't receive a 1099-misc. Only a 1099-b form this year. So, it will be okay to file for taxes next year then?

If you had gross proceeds greater than your standard deduction you should file even though your actual gain is less.