Hi and welcome! I am not new here, I have been gaining and losing the same weight for my entire life. I made the decision to give it another go, and darnit I am sticking to it this time! Forever!
I do think you would be better off knowing where you are starting, weight wise. I don't think getting mentally prepared is a bad idea, but maybe 90 days is a long time...just a thought. How about if, after you weigh in, you make a decision to do something small until your birthday? Say, something like eating a healthy breakfast and going for a walk every day? If you stick with it, after two weeks maybe you could up the ante and do breakfast and lunch, along with walking and some weights? If that goes well, you might be surprised how motivated you are to eat three healthy meals and exercise every day. After all, this is the lifetime goal.
If you made just one or two small (but consistent) changes, you may very well wake up on your birthday and realize you gave yourself a really great gift- ahead start on a lifetime of healthy living. Good luck! You will find lots of motivation here, and I hope that you don't end up ninety days from now wishing you had put that motivation into motion. Get going!