All the information you need can be found on the Grand Canyon admissions Web site. Below is a link to their admissions home page and then one to their application, although you probably won't want to submit any applications until the fall of your senior year.
Admissions page: http://www.gcu.edu/Admissions.php
Application page: http://www.gcu.edu/Admissions/Apply-to-GCU.php
Note, however, that Grand Canyon is a “proprietary" (or “for profit") institution. If costs are a concern of yours (and they probably will be, if you plan to later attend medical school), you might want to also consider public or non-profit colleges. These tend to provide lower price tags and/or more financial aid in the form of grants. (Grants are scholarships that don't have to be repaid; loans—which do need to be repaid—are offered most commonly by for-profit colleges.) In particular, you should consider public colleges in your own state. California has an excellent network of state-funded two-year and four-year colleges. If you are a strong student with good standardized test scores, you may also qualify for “merit aid" at private and public colleges in CA or elsewhere. Most colleges also offer “need-based" financial aid (usually a mix of grants and loans). This type of financial aid is calculated after you have submitted the “FAFSA" form (and, sometimes, an additional form) that asks questions about your family's income and assets. You will have to complete the FAFSA when you apply to Grand Canyon, too, unless you are not seeking financial aid. But do be wary of colleges that are labeled “For profit" or “Proprietary." Although these colleges do offer certain pluses to some students, cost-wise they may not be your best bet.
Note also that, while nursing programs and pre-med programs do overlap, most students who are pursuing a nursing degree do not ultimately become doctors, although some certainly do. Also, some nursing programs are “direct entry" which means that you apply right from high school and start your nursing curriculum right away. These programs will allow you to get your nursing credentials most quickly but they require an important decision at age 17 or 18 and don't leave much time to investigate other majors and careers.
A good way to search for colleges that meet your needs is to use College Confidential's SuperMatch. Go to: http://www.collegeconfidential.com/college_search/ Here you can enter all your preferences for size, location, major, etc. as well as your GPA and test scores, and then the “Results" list will show appropriate colleges that you can research further.
SuperMatch is most effective for students who have SAT or ACT scores. So, as a junior, if you don't have any standardized test scores yet, wait until you do. (If you are taking the PSAT's this month, you should have your scores in December. Although these won't be official SAT scores, you can use them on SuperMatch instead of SAT scores to at least get a rough idea of where you may be admissible.)
You are wise to be planning ahead for college now but don't stress too much over this because you have plenty of time. Be sure to use this time to check out a range of options. It's too early to focus on just one university.