Hello and welcome Jade
i'm giving it a try, even when i'm not the most experienced here
How effective is it to really train online?
Dunno since i've never done it.

How can you tell who will be a good Dom/Master?
Uh oh... i guess only time will tell you if you've found the right one.

Is it rude or bad to refuse an order you do not think you're ready for or do not trust the 'Dom/Master' enough for? Or even for just something you think you should wait awhile before doing it? (such as sending nude pictures over the internet)
No, at first thought i would say it's safe and sane not to do anything you don't feel comfortable with or are not yet ready to do. So, listening to your feelings and act according to them is paramount.
Then, if it is a "good" Dominant, it should certainly be possible to discuss that with him. In fact, if you can't discuss your uncertainties and problems with an order you probably better kick his ass from your life.
About sending pics: Well, i know that this question usually comes right after the "bra size question". Personally i have just two pics that i will send out, and both are as vanilla as can be. But then again, my Master gets to see me naked whenever he wants, so that's no issue.
I would say, if you don't want to send nude pics, that's entirely your decision. If someone pressed me to send nude pics, it would probably set off my alarm bells.

Do you have any advice for a new Submissive, trying to find herself?
Ask questions, go to the chat, read posts and stories and find out what you might like or dislike.
I think the best advice i can give you is to take it slow. No, you're still thinking of going too fast. Slower yet

And of course you can always pm or email me if you think i could help with anything.

Enjoy your journey.