I've split this post into 4 parts;
Part 1 is about preparation.
Part 2 is about building.
Part 3 is about painting.
Part 4 is about aftercare and displaying your models.
Part 1, Preparation.
First of all, when you open the box that contains the brand spanking new model that you've just spent your hard earned cash on, always, always make sure all the parts are there. It's no use starting to build a model, only to find out later on that such and such a piece is missing, or as has happened to me on a couple of occasions, a whole parts sprue is missing! Although this has never happened with any of my Tamiya models so far, I still always check; lesson number 1 learnt!
Second, always thoroughly read the instruction sheet before you begin, familiarise yourself with the order that the model is built in. If you're an experienced modeller like myself, sometimes you can slightly change the order that the model gets built in to make life a little bit easier, but I wouldn't recommend this to a beginner. Sometimes as well, the different stages of the build can be broken down into sub-assemblies to make life easier when you paint the model. This way, you don't have to try and thread a brush into tight gaps to reach a part that needs painting.
The next most important thing is, to organise your work space. Make sure you've got enough room to spread the parts out, so that you can easily swap between sprues as the build progresses, this also helps prevent accidents, like knocking over a pot of paint because you haven't left yourself enough elbow room! What I tend to do, is to have all the parts sprues spread out on my right hand side, the current sub-assembly that I'm working on in front of me, with the instruction sheet just behind it; and finally, I have the glue, paints and paint brushes to my left. This routine has gradually evolved over the years, and it is the one I am now most comfortable with. However, you might want to develope your own routine, i.e. whatever makes you most comfortable.
Next, what tools do you need? The two most obvious are glue and a SHARP craft knife (believe it or not, blunt knives cause worse injuries!). You will also need a pair of fine tweezers to pick up small parts with and maybe a fine razor saw. The next most important thing is the paint brushes you use. I would always recommend investing in the best quality sable brushes that you can afford. The higher quality brushes won't shed bristles when you're painting your model, cheap ones will.
A good, basic model making tool set.
Another usefull tool to have is a Collett Chuck, used for drilling small holes. This is a small tool that looks a bit like a jeweller's screw driver and has attachments that take different sized drill bits.
A Collett Chuck set
Next are tools that aren't essential, but are nice to have; first up is a gadget called 'Helping Hands', this is a weighted base onto which an arm is attached. On this arm you can mount a magnifying glass, and extension arms that hold crocodile clips. I have one of these neat little gadgets and it has made life so much easier. You use the clips to hold parts, and the magnifying glass to better see what you're doing.
My 'Helping Hands' gadget.
Another nice tool to have is a 'Dremel' type mini rotary tool. This can be used to cut, grind, sharpen, polish, drill and a whole lot of other usefull tasks.
Finally, a lot of modellers use airbrushes, but I honestly think these are an unnecessary luxury, as I've found that you can get just as nice a finish with a paint brush, and it's a whole lot cheaper as well!