Okay, I'm not an expert, so I firmly stand by none of what I'm about to say. I'm guessing that you might be like me in that you have a hard time keeping your fat-weight up – I'm 5'5" and I didn't get above 100 lbs until I was almost 20. And, coincidentally, I was about your age when I started working out to gain weight. I also started estrogen around the same age you're planning to. So, again, I'm not an expert, but I can share some of my experiences and maybe they'll be somewhat helpful.
You'll probably want to watch out for exercises that build muscle bulk until you start anti-androgens. What little bulk I did gain didn't last long after I started HRT, but that could also be due to the fact that I pretty much stopped working out. ^_^;; Be especially cautious of bulking up your
trapezius muscles, where your neck and shoulders meet. Trapezii like
this are neither flattering nor easy to hide. Another thing it'd be good to keep in mind is, as I said before, body fat. Too little body fat and no bewbies for you. I was around 110 by the time I started HRT and was quite lean, but I still got up to an A cup more or less (more less than more). It has since increased to a B as I've slowly lost my superhuman ability not to gain weight. How that works out for you, however, will be pretty specific to your body. And as for strength, I didn't notice any big difference in how quickly it dropped off compared to the other times I'd given up my workout routine. Nor have I noticed a big difference in my strength when out of shape. You will not be suddenly unable to open heavy doors by yourself.

I hope some of this is helpful.