I really don't think he/she will be expecting you to talk details. They understand that to do that takes time to build up trust. I'm sure just a really brief summary will suffice.
They will be assessing how it affects you, day to day. What symptoms you have (depression, anxiety, insomnia - whatever), how it impacts your life. They will likely want to talk about whether you have any thoughts of suicide, or self harm. Perhaps also look a little bit into your lifestyle, such as who you have around you for support, whether you put on a brave face and appear the life and soul of the party, or whether you rarely leave the house. Also whether you use drink or drugs, and any other health problems you have. If you have other things going on, such as financial troubles, housing issues, problems in relationships, bereavements, work trouble, victimisation, difficulty with children or parents, etc, etc.
Remember, this is not a therapy session, it is an assessment /evaluation, so they can decide where to go next, and arrange the right sort of help for you. It's not so much about your feelings and emotions, more about gathering the facts. They want to get to know a bit more about you.
Please don't stress about it. I'm sure they allow 90 minutes for everyone, and some people will need all that time, others won't. Some people have massively complicated situations, which might take ages to discuss, other's less so.
Don't over think it. Don't worry about what if's (What if they ask this, what if they ask that). There's no right or wrong answers, it's not a test, just try to be as honest as you can be. The only thing you might want to have in your head is one phrase to get you out of something you don't want to or can't answer, a stalling technique - "I find that really difficult to talk about", and if they push you on it - "can we come back to that later?"
Good luck ;)