For 2018 taxes at this late date, you have to buy, download an install the 2018 software on a full mac or Windows computer.  Then prepare them and then print and "mail" them in yourself at the post office.

You can buy a 2018 download here:

https://turbotax.intuit.com/personal-taxes/past-years-products/

If your mom claimed you as a dependent in 2019, you will not be able to get a stimulus payment.  She would receive the $500 for you as her dependent.

The stimulus payments are based upon the 2019 returns unless you have not filed for 2019 yet.  Then it would be based upon your 2018 return.

Stimulus check calculator