The following things are important about freshman seminars:
- They are very small. Mine only had 6 people, I believe.
- Many of them are not very much work and a fun way to get acquainted with a subject in which you have great interest but see it as only marginally related to your intended feel of concentration.
- Most of them are interesting and a great way to go deeper in some area.
As for whether you should take a seminar, for most purposes the answer is yes. At a place like Harvard where it is quite possible that many of your freshman courses are large lecture courses, having a small class is a huge plus*. This is because you will probably get to know your professor well, which is good in general and because for many summer programs you will need references and/or letters of recommendation. You will also get to know the fellow students well.
As to what kind of seminar you should take, it really depends on 1) what seminars interest you, 2) the rest of your schedule, and 3) what you want to do with your semester. Past experience should matter very little--it might give you an easier semester if you take a course on a topic with which you already have some familiarity, but all that's probably going to do is give you a light first three weeks. Past that I don't think past experience matters a whole lot for most things, since you'll probably do different things with the material than you have had experience with in high school. (For the same reason, don't worry about having less experience than other students.)
Re 1): The most important thing about a seminar is that you find it interesting because that is, in my opinion, the whole point of taking a freshman seminar. (A really good thing to do is look through the CUE guide and prioritize seminars with professors who are known to be good professors. You will likely have a much more pleasant seminar experience and become much more inspired. I would recommend against taking a seminar with a professor who receives below-4.0 CUE ratings.) Re 2) and 3): Another important thing a seminar can do is balance out your schedule. You don't want too many classes that do the same sort of things. I don't think this applies to you as much, but for instance if you are taking three science courses on semester taking a non-science seminar would be a good idea.