Improving your study skills takes time and conscious effort. Whenever I've given lessons in this area to my students, I tell them to take notes on all the areas, and then start addressing them by changing two (or maybe three) habits at a time. It's unrealistic to try to change everything all at once, but if you work on two (or three) things first and then two (or three) more after those first ones are in place, it helps a great deal.
Here are some excellent websites (in no particular order) with lots of ideas:
Read through these sections and take them on in the order they've presented. Don't skip the first two!
It's best to go through this with your parents. They can be your greatest allies, but only if they understand what you're working to accomplish.
Be sure to read through this before making any decisions about what to change in your life.
This one may seem overwhelming, but take it in sections. Don't try to take on all these areas at once.
This, too, has lots of sections. Scan through them before making decisions.
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Here, I'd go through the time management and the study skills inventory before going through the rest. There's also a section on improving test performance.
Here are a couple of sources that might help with test anxiety.
I hope this helps a little more. Thanks for asking.