Been in the same boat a few months ago, where I would panic because entirely new things were coming my way. But as time went on, I realised that the learning/understanding/implementing part wasn't difficult. Rather what was really difficult was to dive head first in the unknown. Now I still have numerous things to work on, in which I am a complete novice, but I know I can do anything I put my mind to.
My advice: if you are given a task and you have no idea what the effort required is, tell this to your manager/lead, and add that you will come back with the ETA by EOD/next day after looking through the issue details. With time you will become better at estimating the ETAs and effort required.