As PPs say, you choose your pharmacy and then complete their application process.
They will need details about your current height, weight, medical history, medications, GP practice. They will also (or certainly should) ask for photos of you showing the shape of your body, and ID so they can verify t your identity and confirm as best as possible that you need to lose weight.
You'll start on 2.5mg, which you take for a min of 4 weeks, if it's working you can stick on that dose or move up to 5mg, then 4 weeks later you either stick or move up.
You still need to change your diet and watch what you eat, the injection does not do it all for you and if you don't change your diet you may find yourself regretting it due to stomach issues.
Mounjaro works by increasing the amount of insulin you produce, making you more sensitive to that insulin and slowing down your digestion so you feel less hungry.
If you put greasy, heavy food into your digestive system, then slow that system down it will not feel good.
So you need to accustom yourself to smaller portions, you will likely feel less hungry, so need to ensure that what you do eat gives you the best fuel to keep yourself healthy, lots of veg, protein, good fats, vitamins, minerals.....
You need to get into the habit of drinking plenty of water if you aren't already.
You also need to ensure you are doing some form of weight/resistance exercise as weight loss often comes from muscle loss and that's even more the case with WLIs, so you need to be building those muscles alongside losing fat.
Generally what people see is an initial bigger loss in the early weeks, then a more reasonable 1-2lbs a week. The big difference with WLIs rather than just willpower is that you don't feel hungry all the time, and aren't thinking about food all the time, which makes it a lot easier to stick to the diet long term rather than lapsing and bingeing and having to constantly restart.