The SCA is divided up into a number of Kingdoms, which are further divided into Shires, Baronies, and other local branches like that. Most people get involved through their local branch, and the system is mainly designed around that. So your first step is usually to find your local branch.

The SCA Home Page has a couple of sections which can help make this easier. Go take a look at the discussion of Locating an SCA Group Near You. This gives a lot of pointers on how to get in touch with your local branch. Once you've found the local branch, contact them and tell them that you're interested in joining. They should be able to help from there.

The SCA Home Page has a page specifically on Locating Your Local SCA Group. This lists several different ways to find it; at least one of them should work.

Yes, with a couple of restrictions. First, you may not be able to participate in armored combat -- due to real-world regulations, many Kingdoms can't let anyone under 18 fight, and none allow fighters under 16. Second, some large SCA events (such as Pennsic) require that you be accompanied by a parent or appointed guardian. Third, you won't be able to take a few specific officer positions yet, again due to legal restrictions.

Other than that -- yes, you are certainly welcome to join in.

The SCA membership form is now available online, in a variety of formats. We can't send it out otherwise; if you need a hardcopy, you'll have to talk to Membership Services.

Put simply, the policy is that you shouldn't push religion into peoples' faces. You specifically can't have any kind of religious ceremony that implies any kind of official endorsement by the SCA of a religion, and should therefore generally keep religious ceremonies out of the main focus of an event. Yes, this isn't very historically accurate, but is necessary, since the people of the SCA are of all sorts of faiths.

Households are unofficial entities as far as the SCA, Inc. is concerned. The rules don't generally provide either guidance or rules about forming them. That means that you have a lot of leeway.

The main first steps are putting together the people, the organization, and the name. You can have anyone in the household that you want; the initial members of the household tend to define its style more or less permanently. In general, you want a group of people who are friends, who share a common vision, and who can work well together in the long run.