You want to do the right thing. And your friend/sister/partner may sense your willing spirit. They yearn to share their story without feeling judged. But they worry about being "too much". They've been told to "get over it,", that they are making a "big deal" or that they are "too sensitive". It happens to them a lot. You may know this. So you want to be different. But the best, first thing you can do for them isn't "I believe you,".
In the past, their story hasn't always mattered. Friends looked away. Folks from church changed the subject. Family walked out. Sometimes, even the abuse became the focal point, instead of their experience. The survivor were urged to press charges or make a complaint. Survivors have often felt the bigness of their abuse, and less their own power as a person.
Yet, survivors need to feel powerful. Power and control was yanked from them in a painful way. Being able to share their story as they would like to gives back some of that power. There is satisfaction in getting to tell a story in their own way. In the re-telling, they get to choose how the story ends. Your "I believe you," is a sentence that not only takes power away but it also ends their story.
The best, first thing a survivor needs is the chance to share their story on their own terms. Wait for them. It may take years. But when they feel safe, they will speak. Then you listen until your gut tells you that they are done. After, you check in on that feeling. "Is there anything else?" you could ask. You wait.
This is hard.
It's easier to barrel in, try to get the whole story, to learn everything. But no matter how hungry you are for connection or answers, their life is not for your consumption.
A time may come in your relationship with them when your gut may urge you to offer, "I believe you,". And if it feels right and true, you can share that. And then again, you may never get that feeling. That's okay too. By this time, the survivor will know you are on their side. And you will both know that while the best, first thing isn't always the easiest, it's often the kindest.