Hello!
1. I use Formula P3 superglue for everything that you mentioned, and I'm pretty sure any generic cyanoacrylate glue will work as well. A little goes a long way; if you use too much the pieces won't bond as quickly, and this is especially noticeable with metal. (I spent a miserable few weeks working on Aurora and Clockwork Angels before noticing this.)
Games Workshop sells a line of Citadel glues, and I couldn't tell you what the difference is between them and the normal stuff. I don't think you need to worry about that as a beginner though.
2. YouTube is probably the best source for painting and hobby tutorials. Other hobbyists have produced detailed how-tos for pretty much anything you can imagine. I also
strongly recommend Autumn Stone's
A Beginner's Guide to ... Beginning series on Hand Cannon Online (link includes an index of articles). The content on that site is mostly geared towards Warmahordes, but the painting and modeling tips there are universal.
The Paint Splatter articles in White Dwarf and Warhammer Visions can be useful, as far as tips for specific colour schemes go. I'm leery of
GW's tutorial books and things, though, because they largely seem to exist to sell the extremely wide and fiddly range of Citadel paints. During the holiday sales last year, for instance, I picked up a $2 ebook that was supposed to have basing tutorials, but was basically nothing but pictures of a new kind of texture paint that you were supposed to use lots of.
As far as magnetizing your Tau models goes, the biggest thing is probably your Crisis Suit models and their special weapons and wargear. I use a Formula P3 hand drill (non-electric, only a few dollars), generic 1/16" - 1/32" magnets (those are their width and height respectively), and a drill bit that's just a little wider than 1/16".
To do it, I clip off the "tabs" on each item, drill a hole where I want the magnet attached, then put a
tiny drop of glue on the end of a whole stack of magnets before sticking that in the hole. The XV8 side is a little easier, since it has a bunch of slots that are almost wide enough for magnets on their own. I use a hobby knife to widen them, before doing the same there. I also use the knife to scrape dried glue off of magnets, when I accidentally get it on them; it doesn't seem to hurt them.
Protip: Use the stack of magnets itself to make sure everything is the right polarity. Attach it to the weapon, and then you know to put the glue on the
other end to magnetize your XV8. Do this every single time, for every piece and mount point. Also, with things like fusion blasters and plasma cannons do a variety of "magnetized on the left" and "magnetized on the right (or top)" pieces.
This will probably work for XV88s and the smaller weapons on Riptides as well. I haven't used any larger magnets than that. You probably shouldn't worry about magnetizing your infantry, at any rate. Pathfinders especially have very specific poses for their arms, and it would get fiddly very fast.
The biggest challenge is making sure everything's even, and not lopsided. Try to keep everything level, and if needed use a pair of clippers to pry the magnet back out. That doesn't seem to hurt them, either.
