I'm terrible with finances but decided that I need to adjust my spending habits. The problem is that I don't know where to start.
This may sound counterintuitive, but the place to start is not in reducing costs, but in gaining a better picture of your spending habits. If you don't do this and try to resuce costs intuitively, you will be taking shots in the dark.
To this end, you need to keep track of all of your spending over the course of at least one month, preferably two. Write down every single purchase no matter how big or small as you make them, you can do this in your notes app. Separate all your purchases into general categories, such as: groceries, eating out, gas/car-related, rent, phone/wifi, entertainment, etc.
If you do this diligently, I can practically guarantee it will give you a new understanding of your own spending habits and help you feel more in control of them. It's important that you actually do it correctly and not try to estimate, the whole point is to get a real, confident picture of your spending rather than a caricature with its pockets pulled inside out and big tears coming out the eyes. Good luck.
*forgot to mention, once you've done your expenses as described above, compare total expenditure to your income and see what your actual margin is.