There is a book recommendation sticky at the top of this forum http://www.polyamory.com/forum/showthread.php?t=1096
It has lots of good books on it... There is a lot to read on line too. I suggest reading about "primary/secondary," "NRE," and anything else that catches your eye when you have a look at the tag cloud.
At your age there should really be no need for primary/secondary relationships. They generally seem to be useful in terms of family/children/joint finances/property. Of which you have none. I would be rather sceptical if I were your age and someone said I was secondary... in terms of time management perhaps one partner can meet up more than another. Anything other than that would be more about a hierarchy and that to me is not fair. Sometimes when someone new comes along the partner that was there before feels a sense of entitlement because they were their first. I would be careful with this as it is valid that they feel some jealousy and be concerned that there won't be enough time for them or they are not loved as much... it takes time sometimes to get through that, but that has nothing to do with someone being better and more of a primary over another.
As to telling parents? Well, they dated at one time before and likely dated several different people at one time... it was called "playing the field" and just "dating." I would think that they will smile knowingly at your arrangement. If you are super open with them then you could talk about poly, but otherwise telling them you are keeping your options open is quite normal and reasonable for your age and stage, not to mention within their realm of understanding... As you grow older and all those expectations of settling into having one partner come down on you, you might want to elaborate then; if in fact you still feel you are committed to having more than one partner by then. You could decide that you just want and can only handle one. If so, then so it is to be. As long as what you do makes sense to you and makes you feel happy.
I haven't read everything others have said here, I hope I haven't repeated what others have said.