Hmm. My one huge mistake when starting out was that I wasn't very assertive. I didn't want to come off as pushy, so I kind of kept my thoughts to myself. Sometimes my partner did something I really didn't like, but I was too shy to tell them about it, so I just tried to deal with it. But then, of course, it ruined my enjoyment of the RP, and kind of killed my enthusiasm for the story. Or I would have a great idea that I really loved, but I didn't want to suggest it because I was worried my partner would think it was a stupid idea. But then I would spend the rest of the story imagining what might have happened if my idea had been used.
Now that I'm more experienced, I definitely recognize that that was a pretty huge mistake. Communication between partners is key. I know I would want my partner to tell me if something was upsetting them, because I care about whether they are enjoying the story as much as I am. I can only imagine my partners probably felt the same way. And the worst that can happen if you suggest an idea is that your partner says no. It's not the end of the world- not even close. But sometimes they'll like the idea, or perhaps help you make it even better, and both of you will like the story even more for it. And that's why I say my biggest mistake was lack of communication. So if I were to give one tip to anyone new to RPing, it would be to not be shy about communicating with your partner out of character.