Contact an Officer Selection Office (OSO). They are the ones with the numbers of slots for the upcomming classes and cutoff dates and all that. People on here can give you an idea, but the OSO will be the one getting you involved. I'm not sure how much you know about the program already so I will leave it at that for now.
As far as being a freshman, you can sign up for the program. Theres a few different programs out there so talking to an OSO will help you determine when you will go. I recommend that you talk to an OSO and get involved as early as possible. This doesnt mean make a commitment, just find out all the info. It is a very long process and theres many steps along the way. Theres also things that the earlier you know, the better. For example, doing something that will get you in trouble and get you disqualified, some type of medical procedure that will get you DQ'd (Lasik, etc...) and other things.
You do not have to be enlisted to get in the PLC program. But you can be. There are different options you have. One guy from our office came around right after he graduated HS. He went to boot camp the following summer and then went into the reserves. That fall, he went to college. He did PLC the following summer.
You can also be a civilian and go straight to OCS. There are a few different options for enlisted guys to get into OCS. All these programs can be detailed a little better by your OSO.
When you sign your contracts for PLC, the only obligation you have is to show up for it and spend 4 weeks there. After 4 weeks (out of the 6 or 10 depending on your program), you may quit. This is referred to as a DOR or Drop On Request. It means you go home and have no committment. You can also complete the first 6 weeks session and elect not to come back for the second 6 weeks. You can also graduate either both 6 week sessions or the 10 week session (again, depending on your program) and not accept your commission. Once you get your bars pinned on though your requirement begins.
So essentially, you can make it all the way through training and elect not to go through with it. If you have no plans on becoming an officer in the USMC at all, then I would higly suggest that you do not go to training. You will hate it and it will show that you dont want to be there.
Hope that helps at all.
Edit: I took too long typing