Hi Sadie and welcome. I just made the decision and threw myself into it not thinking that I would get anywhere near to where I have.
Just take the plunge and start.
Perhaps start by cutting back on takeaways if you eat them, then maybe cut the junk food from the shopping trolley if you buy it and start with a short walk each day and of course if you don't drink enough water then start slowly and build it up.
For exercise when I first started I used to walk up and down my hallway for 15 minutes a day.