What I wish I would have known:
I wish I had known how okay it is to go in not knowing it all. I'm not a scripture mastery aficionado. I memorized a good amount my freshman year of high school and didn't memorize a single one after that. I hated studying the scriptures. I thought personal study would be the literal death of me. The testimony was there, but the whole studying aspect of things was very subpar.
It's the testimony part that's important. You'll learn how to study and how to love the knowledge aspect of things. My mini BoM became a giant cheat sheet of color coded passages for proper occasion, and in time, I learned to absolutely love studying the scriptures. I mean, I poured over them + the bible dictionary + all the old BYU NT text book things + ever other book the handbook allowed that I could find.
Other things I wish I would have known (or believed)--
1. Get over yourself, throw yourself into the work, don't believe members of the ward who tell you you'll be most energized if you only live off of pineapple for 3 weeks straight...etc, etc, etc.
2. With that, though, take care of yourself. Get over yourself but also take care of yourself. It seems contradictory, but it's not. Take care of yourself so that you can do it all for the good, unselfish reasons.
3. Bugs and rats and weird foods don't be a wimp just try it. Bugs and rats both from an extermination standpoint and a food standpoint.
I *maybe* didn't shave my legs for a year of my mission and this was snapped of me when I finally did a few days before going home. I've mainly included it here for you to check out that Crocs tan. Other notes, you'll buy most your PJ bottoms in the Philippines because they have these super nice light weight bottoms that I wish I had brought 10 more of home with me. Secondly, I think my hair was in a bun 80% of my mission. I cut it around the 15 month mark and seriously regretted it because I could no longer keep it in a bun.