Hi. I've brewed for a little over a year and have about 18-20 batches under my belt.
1) I'm not an expert on water. Some brewers recommend you shy away from mineral water, as often you can't get a read-out of its content. Your local tap water, on the other hand, you can likely get a water report. I think at the 'entry level' of brewing, in so far as you want to tamper with your water supply, you'll likely want to achieve two objectives, a) get your mash pH as close to the desired range as realistically possible (I think it's around 5.2--5.3ish), and b) remove chlorine/chloramines from the water. For the first (pH) it is most convenient to use lactic acid (I've found) to add to your water supply, and then test the pH of your mashes and for subsequent batches adjust accordingly. For the second, campden tablets will remove the chlorine/chloramines. Both the tablets and lactic acid will likely be available from you LHBS. If you are extract brewing then you can use R/O water and then the extract contains the minerals the yeast need to be healthy and multiply. (Someone else can please correct me if I'm mistaken).
2) I've heard of hop enhancers, and if I'm not mistaken it/they are used in bittering. I've never used them before, but I would guess that you don't boil them at all, but add them during fermentation. If they were supplied with a kit then check the instructions, and if it's unclear or absent, then contact the manufacturer.
3) I don't know, sorry.
4) DO NOT add extra priming sugar. If you want a higher ABV, then you can add sugar while it's in the fermenter (boil it in some water first), and then let it ferment out. Contrary to what someone said above, it is perfectly fine to use table sugar (sucrose) both to increase your ABV during fermentation, or as priming sugar (for bottling). All the standard sugars, like corn sugar and table-sugar are highly fermentable, so they won't add much or anything in the way of flavor, but will almost entirely ferment into alcohol and Co2. I use table sugar all the time. I also have used corn sugar (dextrose) and found there to be no noticeable difference.
5) Some brewers claim it make the beer clearer to use a secondary. Others say they get perfectly clear beer without using a secondary. I typically secondary if I've got lots of hop debris in the primary. Inevitably I suck some up when transferring to the bottling bucket, and then it jams my bottling wand while I'm trying to bottle -- it's not fun to try and unjam it. If I transfer to secondary as an extra stage then I get a lot less hop debris in the bottling bucket. So I only typically do this with very hoppy IPAs. I recommend you try some brews with only using a primary fermenter, and others using a secondary as well. See what works best for you...