OK, let's see where I can start.
When you buy into
DVC, you buy a certain number of points at a particular resort. You will pay a one time cost for the points (anywhere from about $70 per point resale for OKW, to $100+ dollars for direct from Disney for BLT, AKV, etc). In addition, you will have to pay an annual maintenance fee each year based upon the number of points that you own.
You are buying points that are used to reserve your room at one of the resorts. The cost of travel, food, park tickets, souvenirs, etc is all extra.
There are charts for each resort and villa size that indicates the points required for each room size for each time of the year. You can use your points anyway that you want with some limitations.
Let's say that you buy 300 points at AKV. This year, maybe you decide to get a 1 bedroom. For this example (I'm not looking at a
point chart right now), let's say that you want to stay 1 week and that costs 300 points for the week that you want. OK, you book the 1 bedroom. Then, maybe next year you decide that you want to stay longer. So next year you book a studio for 2 weeks and that also costs 300 points. So you book that.
The way to figure out how many points to get is to figure out which resort that you want. Then figure out what time(s) of you want to travel and the accomodation size that you want. Now look at the points chart for that resort and figure out the number of points needed. If you plan to go every year, buy that many points with a few extra in case the points values are adjusted. If you want to go every 2 years, then buy 1/2 that many points.