I'm not sure why everyone here is getting so down on M for messaging a bunch of girls. Maybe it's not the best thing to just message a bunch of girls just to say Hi, but is it really going to hurt? Maybe some of you should give it a try instead of just condemning it? Worth a shot, huh? And maybe it'll work for some of you like it worked for M here.
In any case, just show some interest in her M. It sounds to me like someone is opening up to you, that's awesome.
She showed you some paintings so talk to her about that stuff. Tell her what you like about her paintings. Ask her if she does anything else like that. Tell her some stuff you like that might be related. Briefly mention something about your own hobbies/interests and see if she says anything. Follow along with the things she talks about and try to participate and actually show interest. Don't just ask her for "updates" on her hobbies. That would be weird. But maybe if you haven't talked in a while you could message her and ask if she has painted anything new lately, just to try start up a new conversation. Don't do it too often or else you'll come across as 'needy' or annoying. Just an occasional "How are you doing?" and things like that so you don't fall out of touch.
Also, if you think the conversation gets a little awkward at some point, apologize and tell her something like how you don't have that many people to talk to and you're 'a little out of practice' or something like that. Hopefully she'll understand.
If you're really wanting to know 'Why?' and it's bugging you not to know, and feel it's an appropriate time to ask, you could tell her you were just wondering why she's talking to you now when she never did before. Maybe she is actually lonelier than she appears. Maybe she was only mean to you before because she felt peer-pressure and she gave in, so maybe now she is trying to change or has already done so. Who knows? You can't know unless you ask. Might as well if you have nothing else to talk about.
Oh, and if you happen to see her around in real life, don't forget to give a friendly 'Hi' as you pass by or whatever. Don't linger for a conversation, unless it looks like she is wanting to talk to you, but just enough that it shows her a sign that you're acknowledging her existence.
I don't know how else to give suggestions about how to have a conversation. Just talk! It doesn't really matter what you talk about as long as you are showing interest in her or it is something she might be interested in. Think of it kind of like an 'exchange'. If she says something, you are expected to say something related in return, and it goes back and forth like this indefinitely.
