First, welcome to TUD @mariam. This is a great place full of people experienced in the disease, parenting kids with it, and helping people who are totally new to it and a bit panicked and overwhelmed. Which is pretty much what everyone experiences right at the start!
Second, this is very early days and it will get better—promise!—but there is a lot to learn. Ultimately you’ll probably want to get her on a pump and a CGM (Continuous Glucose Monitor), which will make things a LOT easier. Learning how much insulin to take for how many carbs is also an important part of it. That differs for everyone, kids and adults alike, and it takes a bit of time to figure out, so right at the start your dr’s are probably just working from general principles before helping you work out a more precise treatment plan. When that happens, though, it will become easier to manage things like cake & biscuits, so don’t feel too dismayed at having to curb that stuff right now—it’s not going to be a permanent situation.
Another thing to be aware of at this stage is something called the “honeymoon” period. This just means that the body may continue to produce some insulin for a while after the disease process starts, and that may explain why sometimes your dosing works, and the next day it doesn’t. Again, it’s a pain, but it’s not unusual and keeping her diet steady and predictable for now (hard as that is with a six year-old!) can help.
I’m sure others will be coming along soon to help with your specific questions. But meanwhile you might have a look at this recent thread from a mom in similar circumstances to yours:
There are a lot of questions and answers there and you’ll probably find some useful tips, as well as perspectives to help you keep your own anxiety about your kid from being too overwhelming. It is doable, and you and your daughter will be ok. And we’re here to help too!
ETA: just want to make sure you’re aware of our Diabetes Terms and Acronyms threads, pinned at the top of the Forum list. They can help you sort through the thickets of specialized vocab pertaining to T1, which can also be a bit overwhelming.