Hello to everyone

I'm new in the software and my goal is to create really basic animation with some objects and upload to YouTube.

I'm stuck in very early stages and I have a few questions:

1. How can I delete frames that I've already created?
2. All objects appear in the first frame, how can I select a specific frame where the specific object I choose will appear? I mean a chronological order of the appearance of objects.
3. How can I exporting the right file type for a YouTube upload?

* I do not paint or design the animation, these are objects that I have already painted before and now they only require animation.

I’ve looked a lot in the search engines and watched a lot of tutorial videos, none of them provided answers to the questions above :/

I'd appreciate any help.
Have a great day