How do you estimate time working on a task? (ticket).
Offer your help. Be nice!
I find it's mostly just a matter of experience - you learn to judge how much work something is going to take, and then you can get a feel of how long things take.
Mostly, it's as @eden mentioned. When starting a new job, it's difficult to say when you don't understand a lot about the system you're working with. If you're solo and need to guess, give yourself plenty of 'padding'. More often than not, you'll end up realizing that your task is bigger/small than expected and is perfectly O.K. to modify the estimation. Just, don't get in the habit of doing that if you can give a somewhat close estimate to begin with.