From: Blank Fraenkel
Hmmm... where shall I start...
1) Can I set up a retail space on my land where I have my "house"?
If you pay tier to Linden Lab, yes. It's worth asking yourself if the location is ideal for a store though. If you are paying tier to another resident, then you will have to pose the question to them, as many have additional zoning rules.
From: Blank Fraenkel
2) If so, can I restrict entry to the "house" part to shoppers?
You could divide your plot up into two and then set different levels of access on each.
From: Blank Fraenkel
3) If I can, how do I put up that wall of red writing that I keep bumping into when flying?
Right click the land, select 'about land' and click the 'access tab'. Here you can restrict entry to either group members only, or listed avatars.
From: Blank Fraenkel
4) Why can't my guests see the picture on my television when I can see it?
If you mean a screen that displays a media stream, it could be one of a number of reasons, it's difficult to answer without more info.
From: Blank Fraenkel
5) Why am I prevented from putting things into my own land? (it says "parcel is full" or some such)
Each parcel supports a maximum number of prims. Once you have reached that maximum allocation, you can not add more without either owning more land in the same sim to increase yur allocation, or removing some of teh current objects. The no. of prims for a particular size plot, along with much more info on land can be found here:
http://secondlife.com/knowledgebase/article.php?id=055From: Blank Fraenkel
6) What's a "prim"?
Prim is short for primitive. They are the basic building blocks in SL. All solid objects are made out of prims that have been shaped and textured.
From: Blank Fraenkel
7) On my first day I was prevented from moving by another person, after I accidentally wandered into his house. Is this allowed? If so how can I obtain that power?
I think, tho am not certain, that any land owner can do that on their own land. You right click the avatar and select 'feeze'.
From: Blank Fraenkel

Why are there cars on the road?
Most roads are on protected land that has auto return switched on. Any objects, including cars, that are left behind get sent back to the owner.
From: Blank Fraenkel
9) How do I go about selling stuff?
http://secondlife.com/knowledgebase/article.php?id=162