In order to build a good looking Gundam it's going to take a few tools. They're not too expensive although chances are you've already got most of these laying around already.

Basics

Pair of clippers:

When you get a Gundam those things the parts are connected to are called runners. A good pair of clippers will make removing the pieces from the runners a piece of cake. Best practice with respect to use is to cut the piece out with two cuts, first removing it from the runner by snipping close to the runner, then removing the rest of the nub (the shit left over) with a closer cut. Not doing so can cause the plastic to compress as it's being cut. On darker plastics this could leave a small lighter area that's tough to remove via sanding.

I use the clippers to the right. The back end which I place toward the piece is pretty flat, helping to alleviate the compression ailment. Back in the day I used nail clippers which work great as well. If you get a pair which includes a good file you can use that to sand down the nub. If not you're going to need one of the following...

Sandpaper and/or a File:

A must have for basic construction. Helps sand down those pesky nubs which both make your kit look a lot better and help the pieces fit together properly.

I'm lazy and a basic metal file for the nubs nearly all the time. As long as I don't go overboard and apply a lot of pressure it usually turned out well. Too much and you could end up with an uneven joint. Sandpaper is generally safer. A medium grit works best initially with a fine one to smooth that out. Careful though, you're likely to sand down a larger area, which isn't a problem if you intend to paint or apply a top coat.

Fine grit sandpaper is also great for cleaning up and smoothing paint.

What I currently use is Testor made pack containing five small wet or dry sheets, 150, 280, 320, 400, and 600 grit. You could probably find this at your local hobby shop for $4

Everyone has their own style so find out what works best for you.

Screwdriver:

Master Grade models sometimes come with screws for the joints (PG all the time) which is where a screwdriver comes in. You want to use a larger screwdriver head to prevent slipping and stripping the screw's head. I have a set of small screwdrivers meant for eyeglass repair ($5-$10 from the local grocery store) which don't get as much use.

Other Useful Tools

Razor:

A really sharp razor, x-acto blade, or scalpel work great for cutting off small nubs. Be very careful though, it's easy to cut into the plastic or into your own flesh. Pictured above is what I use, a small kit including two handles and a ten or so randomly shaped blades. Over kill, for most situations but it was only a few bucks more than a single package.

Tweezer:

While these are of utmost importance when it comes to applying decals & stickers I have them in this section because the come in handy during basic construction. How you ask? Well ever have a really small piece where you could barely hold while trying to sand? Ever have trouble attaching that same piece? That's where these suckers come into play.

Glue:

Click to Enlarge

Inevitably you will encounter a v-fin that won't stay on or accidentally break a piece. You don't really need a huge tube of modeling glue so I like to use a little Super Glue (or in this case, Krazy Glue). A warning though, super Glue can cause fogging on some surfaces, I see it a lot on transparent pieces.