I'm finishing up my sophomore year of high school and really want to join either the Navy or Marines
through ROTC. I am just wondering whether I should
see a recruiter before I start my online application and if I should talk to a recruiter before going to my parent (who aren't happy with the hints that I dropped about joining the military).

You don't need to see a recruiter before hand. You'll be contacted when you've filled out the application by a recruiter so you can do an interview. Also, if you're still a sophomore, you can't start the application online until a year from now anyway.

And you don't talk to a regular recruiter. They don't do anything to deal with ROTC. There are different officer liaisons that will interview you during the scholarship process, etc.

True, you won't have to worry about the application for awhile, but for now you might want to talk to an ROTC recruiter about being interested. They could let you know what you need to improve so that you'll get admitted no-problems.

Actually, you start your junior year, both for ROTC scholarships and the academies. Make sure you take both the SAT and ACT and get the checklist for both the academy and ROTC. If you decide to go ROTC, you have several options. You can go ahead and enlist your junior year and go National Guard (go ahead and start drilling), then apply for ROTC in your state on a guaranteed reserve scholarship, or still apply for a regular ROTC scholarship. Same with the academy. It is all what you want to do and where you want to go to school. The different schools have different programs and some schools even have deadlines.

Also, the post about recruiters is wrong. If you want a federal scholarship, then you talk to cadet command or your college, but if you desire to go National Guard in college (even if you want to commission active when you graduate), then you do see a recruiter. Take here in Georgia, for instance. We have state scholarships solely for high school students who want to attend North Georgia College all four years (or the Georgia Military College first two years and North Georgia last two); you must get a nomination from a representative just like the academies. You see a recruiter for this info.

So look at what you want to do then look at your options based on your state. But all of them start your junior year of high school.

The Navy and Marine ROTC scholarship application has been up for Fall 2009 apps since May 1st. It's here:
https://www.nrotc.navy.mil/application/default.cfm