Hey Maria!
I am Schezeen Fazulbhoy, also a computer engineering student looking to apply for GSOD'19 with Oppia. It is not too late, the deadline for applications is June 28, 2019 at 18:00 UTC. The entire timeline can be found here:
https://developers.google.com/season-of-docs/docs/timeline. That leaves you with a good two weeks to submit a strong proposal.
Even though it is not compulsory, it is recommended to work on a starter project along with your project proposal. You may select one of the topics listed here on the Oppia Wiki:
https://github.com/oppia/oppia/wiki/Season-of-Docs-2019#starter-projects. Also try getting yourself familiar with the existing Oppia documentation, it will surely help give you more insight.
Do not let the impending timeline overwhelm you, I believe two weeks is a good amount of time if utilised correctly. Focus primarily on drafting a strong application proposal, and simultaneously work on the starter project.
All the best to you!!
Regards,
Schezeen Fazulbhoy.