Advice for friend who is new to online fandom?

In terms of specifics... For Doctor Who, it was Gallifrey Base was what swore me off trying anything else other than the Divergent Universe, so it might be best for her to avoid it if she's a novice. If possible, I'd do a quick sweep of each forum to see how people treat one another and how the moderators deal with issues. Gaming forums are often difficult to gauge, but I'd say as a precaution that she keeps her gender to herself. As much as I am loathed to see it happens, there are those who target people based on whether or not they have a Y-chromosome. Unfortunately.

More broadly, the larger the site, the more likely she is to run into problems. It might also be a good idea to coach her as to the differences between interacting online and face-to-face because the social protocols are markedly different from the norm. Encourage her to think about what she posts as well, there's no urgency involved and it pays to clarify as much as you can before you hit reply. Simply acknowledging that there are other points of view (and that they are just as valid) often fixes most issues where people fly off the handle at one another. It's an exercise in not taking things personally and it can be really rewarding for someone with anxiety. Speaking as someone who suffers from it, it has the potential to build confidence and share interests that would be otherwise impossible to do face-to-face. If you can find a place like the DU, it's worth getting involved.

Tell her to come on board the Divergent Universe Express, we welcome kind and caring cyberspace travellers.

People of gentile dispostion need to avoid Gallifrey Base though...