No worries. I asked all those questions when I started hsing in 1982.
Yes, you need to have some sort of plan before you join HSLDA. If you want to withdraw your son soon (as opposed to allowing him to finish the school year), my best recommendation would be to go to your local teacher supply store and buy some stuff. You just want to cover "core curriculum" ("curriculum" is the subjects you're teaching; "instructional materials" are the workbooks or textbooks), which would be English (which includes grammar, composition, spelling, reading), history, math and science. Pick things that look good to you (and to your ds; take him with you).
When you fill out your HSLDA application, you'll tell them what your curriculum is (for example: English, ancient history, basic math and life science); you don't have to list the instructional materials.
Doing this lets you get started without having to decide what on earth your philosophy of education is and whether you should do classical, Charlotte Mason, unit studies, or just-like-school. You can figure that out later.
Yes, HSLDA will help you with the affidavit, but it really isn't as bad as you might think. You will need to choose a name for your school (I recommend *not* using your family name, or the words "home school"). You'll have one student in gr. 6. You are a "full-time day school," NOT a boarding school (even though your student lives at the school, lol; there is a whole different set of rules for boarding schools). Decide whether or not you're "religious;" if so, you're not affiliated with a church. You have one full-time teacher (you). You have one administrator (which can also be you; some people put the other spouse). You'll put your address as the location.
I think there are also some instructions for homeschoolers on the affidavit site...I'm too lazy to go look it up right now, lol, but yes, if you have any questions, ask HSLDA.
Be sure to print a few copies of the affidavit before you submit it.
You will do this again between October 1 and 15 for the 2010-2011 school year.
There will be several conventions coming up in the next few months where you can do some shopping and whatnot; the workbooks that you pick up are fine for the short term, but they are intended to be supplemental (although some teacher supply stores do carry things like Easy Grammar or the Saxon math series or other products which are comprehensive and "real").
It would be good for you to connect with local hsrs. The three state groups (CHEA, CHN, and HSC) all have contacts; CHEA, the Christian group, is the largest and most organized, if you're looking for Christian resources. Depending on where you live, I might be able to make recommendations for a support group.