@claudiawinklemansfringetrimmer congratulations on starting your journey! I started in the 18s and had the same dilemma you did about a healthy BMI feeling way too daunting and unachieveable.
I set my goal weight at 11 stone, just in the healthy BMI bracket...haven't been that weight since my early 20s (currently late 40s).
I remember how huge that 7+ stone to lose felt. And yet here I am, a mere one and a half stone left to go, a whopping five and a half stone lighter.
So go ahead and pick that seemingly impossible goal. And then pick lots and lots of mini goals.
Measure yourself too. I forgot that last part but I have kept one pair of trousers that were my favourites and used to be so tight at the start. They are comically big now.
My advice to any new starters would be:
Don't rush up doses. Stick to what's working for as long as possible. I can't stress that one enough.
If a dose seems to have stopped working try switching injection sites to your arm/stomach/leg. Switching sites increases bioavailability and can have a kickstarting effect.
You may have weeks and weeks and weeks when nothing seems to happen. Keep plodding onwards. Your weight will go down. It all averages out in the end.
Don't starve yourself. Take a well balanced diet seriously but also still do have that thing you're craving. Learning moderation is an important and really freeing moment. My idea of a treat has changed dramatically too.
A lot of side effects aren't caused directly by MJ, they're often caused by lack of nutrition. Low iron is one to watch out for. Do take side effects seriously and read the safety guidelines so you know what to seek medical advice on.
Buy some lovely warm socks! Trust me, you'll need them.
I found that for me it was better to weigh daily and only record the lows rather than having the pressure of one weekly weigh day. This helped me spot the general overall trend and I stopped obsessing over suddenly being a few lbs up.
I also started to think of my weight in general terms eg I'm in the 12s, I'm mid 12s, I'm low 12s.
I also noticed that you'll stick at a new weight bracket for ages, often not losing for weeks, sometimes gaining. It helps to think of this as your body readjusting and resetting to the new weight bracket and taking a rest before carrying on the journey.
Crikey! That turned into a mega essay. Hope at least one thing in there is useful for someone.
Good luck to everyone no matter where you are on this journey.