Freshman year NAVSC is really easy if you pay minimal attention to it. Your other classes will probably be much harder. I had to do almost no work in HS to get by with pretty decent grades (ea6bflyr knows what he's talking about) but it definetly hurt me during my first semester.
Also - if you need it, ask for help before your grades start to slip. Your advisors have a ton of resources at their disposal. Be honest with them from the start so you don't have to explain how you got a D in Chemistry.
PT at my unit is at 0530, which doesn't exactly mesh with the "normal" college student's schedule. Make sure you buy a good alarm clock... splurge for the one that has the battery backup. Find a buddy in your dorm and knock on each others doors or call each other before you leave. Being late will ruin your whole day.
You said you're in decent shape, make sure you do well on the PFA. Remedial PT is not something you'll want to deal with first semester... or ever for that matter.
Lab for us is Thursday afternoons. I usually find it fairly interesting and we've had some great guest speakers... but every once in awhile its a struggle to stay awake. Falling asleep in lab is going to get you special attention that you don't want.
Drill for us depends on whether or not you decide to buy into the sales pitch for the drill team. If that's your thing, you'll spend a lot of time doing it. If not, you'll practice drill for a few hours on the weekends right before the drill competition. If you aren't on the drill team, its not a big commitment at all, maybe 6 hours a semester spread over 3 or 4 weeks.
We also have to be involved in unit activities (IM sports, unit newspaper, aviation club, etc) but those are usually pretty light schedule wise and are a lot of fun so you don't think about how much time they take up. Don't try to sign up for everything though, you'll need more time to study than you think.
Someone already mentioned the drinking/drugs thing. Just avoid the issue altogether. You will be tested for drugs and almost certainly be kicked out if you pop positive. There is pretty much no tolerance for underage drinking at our unit and I hear its the same at most schools, so if you do it make sure you don't get caught.
Oh and even if its not drinking/drug related, go out of your way to avoid getting arrested. They have no sense of humor about these kinds of things even if you hear stories about all the crazy stuff that happened "back in the day".
All and all to survive in ROTC you have to take responsibility for yourself and find a good balance between work and fun.
Good luck.