First, here is a list of supplies that you will need if you want to do this properly.
Sprue Cutter - NEVER twist a part to get it off of the sprue. You should cut it off, scissors can work, but there not great, what is really good are those strong nail cutters, not the little things, but the ones that look like pliers they sell at stores like Rite Aid. A good place to start is here.
http://www.megahobby.com/plasticmodelspruecutter.aspx
Needle Files - Second you will want a set of needle files, for removing the nub left from the sprue off of the part, if you dont it looks bad and can make the parts not fit together correctly.
Here is a nice set
http://www.amazon.com/Needle-Files-Wire-Work-Wrapping/dp/B000SP0JKS
Sand Paper - You are going to want some really high grit sand paper, and some low grit. You use sand paper to get rid of nubs on non flat surfaces, for getting rid of burns and imperfections, for smoothing down glue and putty. You use a low grit for the main issue, and the high grit sand paper to smooth the surface and get a nice smooth finish. Grit is the number of burs on the paper.
I recommend this, I would not use it when there is paint on the kit, because there not the cheap stuff, but they have the low and high grit you need for Gundam kits, though some have been known to use grit up to 2500 (like me)
http://www.scalehobbyist.com/catagories/Paint_and_Construction/TES00008802/product.php?s=4-3&t=2-2&u=2-2&pg=2&ppp=24&sb=stocknumber&so=a&e=0
Filler Putty - Filler Putty is extremely important. Its a putty that, once it hits the air, starts to cure and eventually becomes as hard as the plastic you use it on. You put the putty on seam lines you want to remove. The putty moves into the seam and hardens. You sand down the excess from the surface until you only see the putty in the seam. When you paint it it will look smooth and like one piece, removing the entire seam if done right.
http://www.amazon.com/Squadron-White-Putty-2-3-oz/dp/B000H7L6RC
Spray Primer - Primer is a type of paint you put down first, it adheres to the plastic and makes it so that other paint adheres just as well. Some oils and such in plastic parts can make it hard for paints to stick to the model. Color wise, a Gray primer will cause the colors to be dull, a White will cause them to be more vibrant. If you are going to do a glossy Gundam coat, use white, if you are going to make it flat and more realistic, use Gray.
Paint Types - Last thing for now, paint types. Three main types of paint you will run across, Acrylic, Enamel, And Lacquer.
You will find Lacquer in things like Tamiya Spray paints, it can eat paint that is under it, I find it the BEST to use as a primer and to use as the main coat, deatiling over top of it. Lacquer requires a specific type of thinner, Lacquer thinner, and this thinner will eat ALL other paint. I use lacquer thinner to clean all of my brushes.
Enamel is the most common type you will find. It can use general mineral spirits to thin it. It doesn't really eat other paints, I have used it over top of all types of paint. Its best to use as a base coat or a main color, leaving Acrylic to detail.
Acrylic paint is a water based paint, it can be thinned and cleaned with water. It doesnt attack any other kinds of paint and is ideal for washes and detail work. I do all my detailing in acrylic, as if I make a mistake I can use some water on a rag to just wipe the paint off while not damaging the enamel or lacquer base coat. If I used enamel, I would have to wipe it off then repaint the whole area.