In regards to improving, just find a study book and sit down with it for a bit. You don't have to lock yourself in your room for the whole summer, but spend some time just studying the concepts and doing practice tests, especially during the summer when you're less likely to have homework or other school-related obligations. You'll learn a lot of material over the course of your regular school courses as well.
For the SAT, critical reading is probably the hardest to prepare for. Math is a matter of figuring out what you don't know and learning it, and writing is a matter of learning the tricks of the trade, which a book helps with greatly. (IE misuse of the word "their", misplaced modifiers, etc.) The ACT is more or less the same thing; the only thing that's new is the science section, which really should be called "reading with numbers".
Or at least, this is what worked for me. Got me a good score on all the standardized tests I've taken. Different things work for different people. Find a study method that works best for you, and stick to it.