not a newbie, but gonna speak as someone with over a decade of tumblr rp experience. my biggest advice for anyone starting out is a two-parter:
Roleplay is communication.
A huge pitfall of this hobby, not just on tumblr but in general, is this weird separation of roleplay from communication or this misunderstanding of where roleplay lies within communication. Roleplay is perhaps one of the most communicative hobbies out there, as its fundamental is based on, effectively, the grown-up version of playing pretend. At the end of the day, you're talking with someone. That's what the hobby is.
What this means is that the standards of communication still apply, and that roleplay is healthier if you step back and consider communicative etiquette. You need to check for boundaries and make your own heard. Always remember that there is a real person behind the screen. Read rules. If you are not interested in things potentially not going your way or having to acknowledge what another person may want out of roleplay, you aren't going to enjoy the hobby. You need to respect the other party's pace, and request respect for your own. Do not assume over-familiarity - mutuals doesn't automatically equal friends, and an rp friend is not the same as a friend who knows your trauma. If you're unsure or don't feel confident, talk it out.
I've noticed overtime that people seem to forget that rp is a form of communication, or, alternatively, that rp is an acceptable substitute of other forms of communication (like the sort of etiquette within an established friendgroup). IMO this has lead to a lot of the drama and stress that Tumblr RP is known for. I think remembering that roleplay is communication, but it's own unique form of communication that requires acknowledgement and respect of the real people engaging in it is critical to having a good rp experience.
2. You've GOTTA curate your own experience.
Tumblr RP is messy. You may feel obligated into following someone back, even if you're not sure about them. You may get overwhelmed if you fill your dash with strangers or feel impulsed to interact with things you don't want to for the sake of appeasing an illusory crowd. If you aren't using tumblr's tools to ensure the rp experience you want, you are going to walk into a lot of unneeded discomfort and maybe drama.
My personal advice: thoroughly vet someone from afar before you follow. Do you like the vibes? Do you have trusted friends you can get a second opinion from? Are you even interested in the other party's writing or character? Really, truly think about it. I promise you, trying to get more interactions is not worth putting yourself in spaces where you are uncomfortable. Be liberal with unfollows and blocking - it isn't an "attack" or an "admission of guilt/wrongness", it's you choosing how to engage the hobby in the way you want to. Filter tags. Consider if there's anything in the hobby that you don't actually like doing, you just feel pressured to.
A strictly curated tumblr rp experience is, frankly, the best tumblr rp experience (imo).
Sorry for the long ask, but wanted to help out by dropping in some RP Veteran advice as it were. Thank you for this blog!
I couldn't have said it better myself.