Jade - first of all, congratulations! Gaining admission to law school is a great accomplishment and doing it at age 18 is especially impressive. Well done!
It sounds like you want to become familiar with basic vocabulary and concepts that you'll encounter as a first year student. If you want to try to accelerate your learning curve, consider picking up a couple of "Nutshells." Nutshells are short books that provide top-line, readable summaries of key principles in diverse areas of the law (e.g., criminal law, constitutional law, etc.) They are generally fairly inexpensive (comparatively speaking). I suggest waiting to buy textbooks, hornbooks (treatises) or study outlines unless you know for sure what books your professors want you to use.
When I was a first year law student, Scott Turow's "One L" was popular - you might find it interesting as well.
You'll be doing a lot of reading and studying when you start school in the fall. Consider 'taking a break' this summer and reading for pleasure when you read, rather than trying to keep pace with or outpace your classmates. Alternatively, consider reading material that will help strengthn your analytical and writing skills. Reading, digesting and cogently summarizing complex materials can help prepare you to read cases, extract relevant legal principles, and identify critical facts that help form the basis for different results.
As an alternative to learning by (exclusively) reading, you might enjoy spending some time in your local courthouse, listening to trial courts resolve motions or hear cases. Courtroom observation may make your formal studies of civil procedure, criminal procedure, family law, evidence, etc., more interesting and meaningful. Another thought - judges usually have law clerks who are typically recent law school graduates. Depending on the rules in your courthouse, it may be possible for you to seek an introduction to a law clerk to discuss his/her law school experience and any success tips s/he might have.
Again, congratulations and best of luck!